Six patients with the intermediate form of cystinosis are described. Two have new mutations not previously described. The disease occurs due either to the combination of one mild mutation and one which is known to cause nephropathic cystinosis or to homozygosity for a predicted mild mutation. Partial phenotypic correction of cystinotic fibroblasts by transfection with normal cDNA or a cDNA derived from a mutation causing intermediate cystinosis is demonstrated.