We compared quantitative urinary cystine values in stone-forming family members and unaffected relatives based on an index stone-forming patient. A family pedigree of 39 members was identified. Quantitative urinalyses and serum biochemical profiles were collected with the subjects on similar diets over 72 hours. Detailed medical histories were obtained. Three family members are stone-forming cystinuric patients with a mean urinary cystine excretion of 496 mg/g of creatinine per day. Six persons without a history of stones (mean age 50) had a mean urinary cystine excretion of 364 mg/g of creatinine per day. Thirty-four family members had elevated urinary cystine concentrations (mean 151 mg/g of creatinine per day), and only two family members had normal urinary cystine excretion. Fifteen subjects with elevated urinary cystine values (> 150 mg/g of creatinine per day) had no history of urinary stones despite a mean age of 44 years. Urinary electrolytes and volumes were similar in the stone-forming and non-stone-forming groups. Thirty-nine per cent of the persons from this family pedigree had elevated quantitative urinary cystine concentrations and no history of urinary stones despite a mean age of 44 years. Acalculous cystinuria is an elevated urinary cystine concentration without evidence of urinary stone disease. Despite abnormal urinary cystine excretion, acalculous cystinuria does not necessarily culminate in clinical urinary stone disease.