Studies of four stoneforming cystinuric subjects from three unrelated pedigrees indicated that each was heterozygous for two of the three described mutant genes producing cystinuria ( I, II, III). Their genotypes were I-II, II-III, I-III, and I-III, respectively. These doubly heterozygous patients were phenotypically indistinguishable from cystinuric homozygotes of genotype I-I, II-I, or III-III. The data provide the first direct evidence that all of the known mutations responsible for the genetic heterogeneity in cystinuria are allelic.