Structural and ultrastructural study of the ovary in childhood leukemia after successful treatment. Ovarian biopsy specimens from ten girls (three postmenarcheal) who had undergone antiblastic treatment for acute lymphoblastic leukemia (ALL) and were in complete remission were examined by light microscope. The biopsy specimens from four of these patients (three postmenarcheal) were also observed by electron microscope. The structural and ultrastructural analysis showed a reduction in the number of follicles which were otherwise normal. No follicles were found in the thin sections from two of the three postmenarcheal girls, whereas normal follicles were observed in the third. The cortical stroma showed moderate to severe signs of fibrosis and changes of capillaries. All of these alterations were more evident in patients where ALL was diagnosed at an older age and this finding suggests that they are at a higher risk for low fertility or early menopause. 