This paper describes the clinical history of two male nontwin siblings with late-onset cystinosis, a variant of cystine storage disease. The diagnosis was established clinically and confirmed by measurement of cystine concentrations in leucocytes and skin-fibroblasts. Both patients presented with an incomplete nephrotic syndrome and renal biopsy showed, in addition to lesions of polykaryocytosis, a picture of focal and segmental glomerular hyalinosis. Renal function was stable in one patient over a follow-up period of two years; the other patient progressed toward terminal renal failure and was successfully transplanted.