In order to determine the incidence of cataract surgery following total body irradiation (TBI), questionnaires were mailed to 173 surviving patients who had received single fraction TBI for haematological malignancies. All patients had undergone bone marrow transplantation at the Royal Marsden Hospital, Surrey, between 1977 and 1991. Replies were received from 135 patients (78%). Fifty-four patients had required cataract surgery. The probability of requiring surgery for cataract at 2, 5 and 10 years post TBI was 5%, 39% and 58%, respectively. No cataract surgery was performed at less than 2 years after the time of TBI, and 12 years is the longest interval, prior to surgery, recorded so far. From a number of potential risk factors, those found to predict independently for cataract surgery, and their relative risk (RR) factors, were: cranial radiotherapy preceding TBI (RR 4.2 for patients irradiated in year prior to TBI, 3.3 for others irradiated); skull dose (RR 2.3 for patients over 25 years at time of TBI); TBI dose rate (RR 2.1 for dose rate > 3.5 cGy/min). An additional 31 patients (22%) reported the presence of cataracts which had not yet required surgery.