1. Two cases of homocystinuria at ages 14 and 21 are described, in which the typical features of the disorder were found: subluxation of the optical lenses, mental retardation, seizures, pes cavus, blotchiness of the skin. Additionally, emotional lability was prominent and breath-holding attacks had been present in infancy. Neither had suffered any apparent vascular occlusive episode. Prominent in both of the children described was generalized osteoporosis and narrowing of joint spaces. 2. Homocystine and methionine were found to be elevated in urine, plasma and cerebrospinal fluid.