In a period of 6.5 years, acute leukaemia was diagnosed in 140 children at our hospital: 137 children had long bone radiographs and 45 patients had bone lesions. Eleven of the 115 patients who had skull radiographs had osteolytic lesions and another four had wide sutures. No patients had bone changes at relapse or at cessation of 3 years' successful therapy. In acute lymphoblastic leukemia, the frequency of osseous lesions tended to be higher in patients in sub-groups with a more favourable prognosis. The duration of remission and survival times were higher in patients with "leukemic" long bones than in those without them (p less than 0.10 and less than 0.05, respectively). Changes in the skull could not be related to the outcome. We found no abnormalities in the bones of the eight patients with acute non-lymphoblastic leukemia.