The etiology of keratoconus remains obscure; yet, this disorder affects an estimated 50 to 230 individuals per 100,000. In the past, keratoconus was viewed as a discrete corneal disease; however, the association of atopic disease, contact lens wear, and other factors with keratoconus has led some authors to suggest that keratoconus is not itself a disease but a sign and that its etiology may result from more than one factor (1,2). In an attempt to support this idea as well as to suggest a collagen tissue abnormality as one possible etiologic factor in individuals without other obvious signs of a collagen disorder such as Marfan's syndrome or Ehlers-Danlos syndrome, the authors undertake this pilot study to look for the association of keratoconus, hyperextensible joints and mitral valve prolapse. Should a strong association be found, the authors propose a future systematic study of patients with all three disorders, extending the work-up for connective tissue abnormalities as well as initiating studies of the corneal collagen by collagen chemists in an attempt to identify distinct abnormalities, such as abnormal collagen cross-linking, in the individuals in this group. Ninety consecutive patients presenting to the Emory Eye Clinic, male or female between 15 and 55 years of age, who are diagnosed by slit lamp microscopy, photokeratoscopy or keratometry to have keratoconus and who are willing to undergo testing for hyper-extensible joints and echocardiography will be enrolled as cases. Ninety patients who have no sign of keratoconus on the above tests and who are race, sex and age-matched to individuals with keratoconus and who are willing to undergo testing for hyperextensible joints and echocardiography will be enrolled as controls. An observer masked to eye and heart condition will undertake five joint measurements to confirm the presence or absence of hyperextensible joints. Another observer masked to eye and joint condition will perform cardiac auscultation looking for signs of mitral valve prolapse. An experienced technician will perform m-mode and 2 dimensional echocardiography. A Cardiologist who differs from the one who performs auscultation and who is masked to eye and joint condition as well as outcome on auscultation will read the ultrasounds, diagnosing the presence or absence of mitral valve prolapse according to established criteria.