Eighteen patients with nephropathic cystinosis who were younger than 42 months and 11 patients 4 to 31 years of age were entered into a double-masked, randomized, placebo-controlled trial of topical cysteamine eye drops between November 1985 and September 1989. Eight of the younger patients and 2 of the older patients showed marked clearing of corneal crystals in one eye compared with the fellow eye. When the code was broken, all 10 patients were found to have received cysteamine eye drops in the improved eye. Of the remaining 19 patients 4 were unavailable for follow-up. In 15 patients no marked difference was noted between the two eyes. Eight have presumably been in the protocol for too short a time and several have been poor compliers with the therapy. These results not only demonstrate the potential for primary prevention of corneal crystal deposition but also, for the first time, offer the possibility of reversing the corneal complications of cystinosis in older patients.