IT HAS BEEN more than five years since Ghadimi first described histidinemia; while the disease is apparently caused by an enzymatic defect, absence of histidase, there is no ready explanation of the relationship between the increased level of this plasma amino acid and the speech defects found in most cases. What has not been appreciated, perhaps, is the fact that the mental retardation observed in some of these children may be familial rather than due to the histidinemia. Two families which have come to our attention within the past five years varied considerably in both the biochemical characteristics and clinical findings. In the first family, a girl 13 years of age, is only slightly mentally retarded, but has a severe emotional problem. Her fasting blood histidine level was 10.4 mg/100 ml. Her younger sister has near normal intelligence, repeated kindergarten, has a marked speech defect, but has a fasting histidine