Eighteen pediatric patients with cystinosis were treated with cysteamine. Treatment was started at a variable age, between 10 months and 7 years, and was continued for 6 months to eight years. Results were evaluated on renal function changes and growth. Whereas 65% of patients with cystinosis develop terminal renal failure before the age of ten years, three (16%) of our 18 patients given cysteamine started dialysis before the age of ten and all three had first received cysteamine only after 4 1/2 years of age. The plasma creatinine level was also lower in treated patients as compared with a control multicenter group. Growth was also significantly improved by treatment, especially in those children treated before the age of 26 months who were taller by 2 SD at age five and 2.5 SD at age eight than untreated controls. Some children who complied strictly with the treatment regimen had an normal stature (- 1 SD) at ten years of age. In conclusion, the effectiveness of cysteamine seems obvious in this group of patients if rigorous compliance with the drug dosing schedule is achieved.