Sex chromosome abnormalities in human stomach and lung cancers from 33 male patients were examined by Southern blot hybridization with pDP34 DNA probe, which recognizes X and Y chromosome-linked restriction fragment length polymorphisms (RFLPs). Contrary to the recent cytogenetic observations showing high incidence of loss of the Y chromosome in solid tumors, loss of the Y chromosome was observed in only 3 of 21 stomach cancers and 2 of 12 lung cancers. Gain of the Y chromosome was found in one of 12 lung cancers, but not in any of the stomach cancers. No X chromosome abnormality was found in any of these 33 stomach cancers and lung cancers.