The ovaries of children with abdominal tumours were studied in 12 autopsy specimens. Ovaries from 25 children who died in accidents or after a short acute disease served as controls. All ovaries from normal children showed follicle growth, but follicle development was inhibited in 67% of the children with abdominal tumours. The effect of treatment with cytotoxic drugs and/or abdominal irradiation on ovarian morphology was investigated. Normal ovaries were found only in children who had received no chemotherapy or a short course. All patients who had been treated with radiation therapy either alone or in conjunction with chemotherapy had severely damaged ovaries: follicle growth was inhibited in all cases, and the number of small non-growing follicles was markedly reduced in most. It is argued that abdominal irradiation might impair follicle development as well as destroy small follicles