The ovaries of leukaemic children were studied in 31 specimens obtained at autopsy. Twenty-eight ovaries from normal children of the same age who died from misadventure served as control. All ovaries from normal childred showed follicle growth and contained several large antral follicles. Follicle development was inhibited in all ovaries of leukaemic children; 22% showed no follicle growth (quiescent ovaries), and in the ovaries in which there was follicle development, the number and size of antral follicles was significantly smaller than in the control. All children had been treated with cytotoxic drugs, the duration of the treatment being correlated with the stage of ovarian development. The ovaries of children treated for only 1 week were near-normal, while those treated for more than 2 months showed inhibition of follicle growth. It is argued that the disturbance in follicle development is an effect of the cytotoxic drugs, and not an effect of the disease itself.