Cystathioninuria and hyperglycinuria both occurred in a male Ashkenazi-Jewish Infant. Both traits were dominantly inherited as the heterozygous forms of two independent mutant alleles. The patient was thus a double heterozygote. Renal iminoglycinuria appeared as an autosomal recessive trait in the mother of the proband and In four of her sibs. This condition is the homozygous form of the trait that presents as hyperglycinuria in the heterozygote; it is a benign inborn error of amino acid transport. Cystathioninuria is probably also a benign trait. Knowledge of the precise phenotypes and their significance were of practical value in counseling the family.