Cataract development in bone marrow transplanted (BMT) patients was studied prospectively. There were 61 children, transplanted before the age of 18 years, who survived more than 1 year after transplantation. Patients transplanted for leukaemia (n = 43) were conditioned before BMT with cyclophosphamide (Cy) and total body irradiation (TBI). Patients with severe aplastic anaemia (SAA) (n = 9) only received Cy. None of the patients with SAA developed cataracts. All children with leukaemia, who were followed for at least 3 years (n = 37), developed lens posterior subcapsular cataracts (PSC). Cataract extraction was performed in 28 eyes, on average 5.1 years (range 3-9 years) after BMT. Postoperative corrected visual acuity was similar to that before BMT. The majority of cases needed laser capsulotomy within 2 years after cataract extraction. TBI seems to be the main cause for the high incidence of cataract after BMT. A relationship to steroid administration could not be proved, but a contributory effect is not excluded.