Restriction of sodium intake has been shown to decrease urinary elimination of cystine in adult subjects with cystinuria. This simple therapeutic recommendation may be particularly useful in pediatric patients whose compliance with high fluid ingestion and repeated doses of alkali is usually poor. We studied the effect of sodium intake in 5 cystinuric children (3 males) aged 5.9-9.3 years. Urinary excretion of cystine (x ± SD) was determined at the end of two sequential 1-week periods in which sodium content of diet was modified. Reduction of sodium intake brought about significant decrases in natriuresis (6.0 ± 2.1 vs. 1.5 ± 0.5 mEq/kg/day, p < 0.03) and urine cystine concentration (328.0 ± 190.7 vs. 14.1 ± 7.4 mg/l, p < 0.02) while urine volume output remained unchanged (58.0 ± 36.0 vs. 70.3 ± 33.0 ml/kg/day). These findings confirm that elimination of cystine is highly influenced by sodium intake in cystinuric children and suggest that a low sodium diet may play a key-role in the treatment of pediatric patients with cystinuria.