Background/aims: Cystinosis, a rare autosomal recessive disease, manifests with renal Fanconi syndrome during the first year of life. Interstitial damage is a major cause of renal failure in patients with cystinosis. We presume that albuminuria contributes to the development of renal failure in these patients. The aim of this study was to examine whether the administration of ACE inhibitor enalapril diminishes albuminuria in patients with cystinosis. Methods: Five patients with cystinosis aged 4 - 9 years were studied. All patients had Fanconi syndrome and were treated with cysteamine. Median creatinine clearance was 48 ml/min/1.73 m(2) ( range 21 - 61). The excretion of albumin and alpha(1) microglobulin as well as arterial blood pressure and serum creatinine were evaluated before and at 3 months on oral administration of enalapril (0.15 mg/kg once daily). Results: At 3 months on enalapril, albuminuria decreased in all patients ( 1,042 vs 629 mg per 24 h, p < 0.05). The median reduction of albuminuria was 43% ( range: 4 - 72%, p < 0.05). Urinary excretion of alpha(1) microglobulin remained constant. Systolic blood pressure decreased from median 110 - 100 mmHg ( p < 0.05), while diastolic blood pressure remained stable ( median 60 mmHg). Creatinine clearance decreased from median 48 - 45 ml/min/ 1.73 m(2) ( p < 0.05) and returned to previous values after discontinuation of enalapril. Conclusion: ACE inhibitor enalapril diminishes albuminuria in patients with cystinosis and might be used in these patients in order to slow the progression of renal insufficiency attributed to proteinuria.