Ten patients with familial hyperlysinemia with lysine-ketoglutarate reductase deficiency, identified through newborn screening programs or family surveys, were selected for review. Ages ranged from 2 to 24 years when last examined. A low-protein diet had been administered to two patients, which reduced the plasma lysine levels from 20 mg per dl or more to about 12 mg per dl. The rest were untreated. Mental development was judged normal or above average in nine. Mildly subnormal performance in three was considered appropriate to family and social background. No adverse mental or physical effects could be attributed to the hyperlysinemia. A normal child has been born to a mother with hyperlysinemia, indicating that the fetus may develop normally despite exposure to high lysine levels.