Familial hyperlysinemia is an autosomal recessive disease caused by a defect in the bifunctional protein α-aminoadipic semialdehyde synthase. The two associated enzyme activities, lysine-ketoglutarate reductase and saccharopine dehydrogenase, normally initiate the degradation of lysine by removal of the ε amino group. In familial hyperlysinemia, both activities are reduced to 10 percent of normal or less, causing hyperlysinemia and lysinuria, frequently accompanied by a relatively mild saccharopinuria. The condition appears to be benign. A variant, saccharopinuria, has been described in which 30 percent of lysine-ketoglutarate activity was retained and the saccharopine dehydrogenase activity was undetectable. Saccharopinuria was prominent, exceeding the associated lysinuria. Experience with the variant is too limited to know if this metabolic abnormality causes disease.