Although the sex of an individual confers one of the greatest of the known risks for contracting leukaemia and lymphomas, very little attention is paid to these risks. It is the purpose of this paper to stimulate further research in this area. The sex rate ratios are presented for the commoner haematological malignancies. The male excess in the lymphoid cancers is most marked in the youngest age group in non-Hodgkin's lymphoma and Hodgkin's disease, while acute lymphoblastic leukaemia shows equal sex ratios in the childhood peak. Both chronic lymphocytic leukaemia and lymphocytic lymphoma display an unusual pattern, hitherto undescribed, with a large male excess specific to the 40s and 60s age groups. The myeloid sex ratios are all characterized by slight female excess in early adulthood followed by marked male excess. The reasons for these patterns are discussed.