OBJECTIVE To assess the causes for delay in the diagnosis of homocystinuria. DESIGN Clinical and laboratory data were collected from patients diagnosed as having homocystinuria due to cystathionine synthase deficiency, with special reference to the ages at which the patients had their first major signs of the disease, ectopia lentis was established, and homocystinuria was diagnosed. SETTING University hospital in the Netherlands. SUBJECTS 34 patients (18 males) in whom homocystinuria due to cystathionine synthase deficiency was diagnosed in the period 1970-94. RESULTS Among 34 consecutively detected homocystinuria patients the mean age at diagnosis of homocystinuria was 24 (range 1-61) years. Despite frequent ocular manifestations, serious complications in the vascular, skeletal, and central nervous systems, and repeated examinations performed in these patients by clinicians of various disciplines, there was a mean delay of 11 (0-43) years between the first major signs of the disease (at mean age 13 (1-40) years) and the ultimate diagnosis of homocystinuria. Even when ectopia lentis was diagnosed (in 26 (76%) patients, mean age 18 (1-50) years), this did not lead to adequate biochemical analysis for homocystinuria at the time of detection, causing a mean diagnostic delay of 8 (0-24) years in these patients. CONCLUSIONS Three factors should have precipitated the diagnosis of homocystinuria: early recognition that unusual myopia (high, very high, abnormal progressive, or at young age) was caused by subluxation of the ocular lenses; awareness that the occurrence of myopia combined with systemic complications ("myopia plus") might be due to homocystinuria; and appropriate biochemical investigations carried out in patients with ectopia lentis and in their siblings.