Formation of posterior subcapsular cataracts is a known complication of systemic corticosteroid therapy. However, the relation between steroid dosage, cumulative dose and type of steroid on one hand and the subsequent formation of cataract on the other is unclear. We carried out a study to determine the incidence of posterior subcapsular cataracts in 64 children who had undergone renal transplantation and to attempt to determine what factors were associated with cataract formation. Cataracts were detected in 17 (26%) of the patients. The steroid dosage, cumulative dose and duration of therapy were not associated with cataract formation. There was a significant difference in the distribution of HLA-CW3 antigen between the patients with cataracts and those without cataracts. The reason for the link between corticosteroid therapy and formation of posterior subcapsular cataracts remains unclear.