The frequency and pattern of obesity in survivors of acute lymphoblastic leukaemia (ALL) was examined in a retrospective analysis of height and weight at zero, two, and four years from diagnosis in 40 children (19 boys and 21 girls). The children had been treated according to the Medical Research Council protocols UKALL VIII and X, both of which included cranial radiotherapy at a dose of 1800 cGy. Body mass index (BMI), determined as weight/height, was used as a measure of fatness. The BMI Z scores were calculated for each patient from standard tables. The ALL group was compared with a control group of 18 age matched children who had received chemotherapy but no radiotherapy. Changes in BMI between diagnosis and two and four years later were analysed by paired t tests. Mean BMI Z scores at diagnosis were similar between ALL boys, ALL girls, and the control group. Two years after diagnosis the ALL group, particularly the girls, showed a significant increase in BMI. By four years BMI had decreased slightly in the ALL boys, but had increased still further in the ALL girls with 57% having BMI Z scores greater than 2. In the control group BMI increased, but not significantly, at two and four years. It is concluded that the obesity seen in patients treated for ALL is more pronounced in girls than boys, and that cranial irradiation is an important factor.