Children with hematological malignancies (n = 33), severe aplastic anemia (SAA, n = 7) and other non-malignant diseases (n = 4) were followed for cataract development after bone marrow transplantation (BMT). The children with hematological malignancies were subjected to total body irradiation (TBI), 10 Gy, in one session with no shielding of the eyes as part of their conditioning regimen before BMT. The children with SAA or other non-malignant diseases received either no irradiation before BMT or a reduced dose, 8 Gy, with shielding of their eyes. After 3 years all patients who had undergone BMT for hematological malignancies had developed lens opacification. No patients in the other groups, without leukemia, showed any sign of cataract development. There was no relationship between steroid treatment for graft-versus-host disease and cataract development. No relation to age of onset of treatment or to the sex of the patient and cataract formation was seen. It seems evident from the present study that TBI given in one session was the main cause of cataract development after BMT.