“Iridology, apparently, can only discern those things that cannot be verified or falsified. What you end up with is a medical cold reading – similar to what a mentalist does to create the illusion of mind reading or psychic powers. While “reading” the iris the iridologist can ask about certain health issues. If they are present, that is used to validate iridology. If absent, then the subject simply has a susceptibility for the missing problem…Iridology is an excellent example of pseudoscience in medicine, displaying many of the core features. It was invented by one individual based upon a single observation. It follows a pre-scientific notion of biology – the homunculus model. It lacks any basis in anatomy, physiology, or any other basic science…Anyone using or promoting iridology is, therefore, a pseudoscientific practitioner. Any profession that endorses iridology is not science-based and should be looked upon with suspicion.” Steven Novella MD, Science Based Medicine (21st December 2011)

“The concept of iridology is biologically implausible, not based on generally accepted scientific principles, and there is no evidence its methods are efficacious in detecting or treating human disease.” Campaign for Science Based Healthcare (2009)

“…iridology is of no value in diagnosing patients. Iridologists had no better than chance percentage of correctly diagnosing a patient and would often give a false-positive, diagnose them with a disease that they didn’t have, which could cause a patient mental anguish as well being financially expensive. More dangerous of course are false-negatives, not picking up on a disease they suffer from. For these people, seeing an iridologist could delay the proper medical diagnosis of their disease which, worst case, could prove fatal.” Skeptical Say How blog (19th January 2010)
 

What can the eyes really tell?

Quackwatch article.

"Research suggests that iridology is not an effective method to diagnose or help treat any specific medical condition." InteliHealth (May 2004)

"What is most peculiar about the iris is that each is unique and unchangeable, so much so that many claim that the iris is a better identifier of an individual than fingerprints". (The Skeptic's Dictionary)

"Besides repeated failures and an evidence vacuum, there is one other essential problem which iridology faces: iridologists agree that the markings of the iris change to reflect changing states of health. Yet the pattern of a person's iris, like their fingerprints, remains the same their whole life long, which is why, like fingerprints, the markings of the iris can be used for biometric identification." Skeptic Wiki (The Encyclopaedia of Science and Critical Thinking)

"Iridology is an alternative medicine practice in which patterns, colors and other characteristics of stromal fibers of the iris are examined for information about a patient's systemic health. Practitioners match their observations to iris charts which divide the iris into many zones believed to correspond to specific parts of the human body. Little if any rigorous scientific evidence exists confirming any such link between aspects of the iris and a patient's state of health and there is no recognized causative mechanism for any purported correlation." Includes a look at the history of iridology, methods used by iridologists, support for iridology, criticism of iridology, and scientific research into iridology. (Wikipedia)

“Iridology has been tested many times and has always failed. Nonetheless, it is still popular, particularly in Belgium and France.” James Randi (An Encyclopaedia of Claims, Frauds, and Hoaxes of the Occult and Supernatural)

Reports of people harmed by iridology.

Including the Bates System and Vision Therapy. (Quackwatch)

Iridology is not a useful diagnostic technique, producing results no better than guessing. Its use is therefore misleading, with a large risk of false-positive and false-negative diagnosis leading to inappropriate treatment or failure to treat. (Bandolier)

"Iridology is a method of detecting tiny defects or impurities in the iris. Their location and colour tell the iridologist which organ is endangered. Iridologists make several assumptions that are clearly out of line with our knowledge of anatomy and physiology." Edzard Ernst, British Journal of General Practice (May 2007)

"Iridology is considered to be scientifically implausible, does not meet professionally accepted standards, and lacks research and literature for efficacy and/or utility." Clinical practice guideline from American Speciality Health Inc. [ASH] (February 2006) [pdf]

Concludes that iridology was of no value in diagnosing the cancers investigated in this study. Journal of Alternative and Complementary Medicine (June 2005)

"Controlled studies reveal that iridology is of no use whatsoever for the detection of cancer and other diseases in the stomach, intestines, kidney, lungs and heart…and it is concluded that this type of alternative medicine is not harmless." Dan. Medicinhist Arbog. (2003)

"Iridology is not merely worthless. Incorrect diagnoses can unnecessarily frighten people, cause them to waste money seeking medical care for nonexistent conditions, or steer them away from necessary medical care when a genuine problem is overlooked." Article by Stephen Barrett, CANOE (25th October 2000)

E. Ernst, Archives of Ophthalmology (2000)

"The validity of iridology as a diagnostic tool is not supported by scientific evaluations. Patients and therapists should be discouraged from using this method." E. Ernst, Forsch Komplentarmed. (February 1999)