Inhibin is a dimeric (alpha and either betaA or betaB subunit) protein involved in the regulation of fertility. Inhibin A and B or its free alpha subunit monomer are elevated in various gonadal and prostate cancers and thus serve as useful diagnostic tools for certain ovarian tumors either as a serum marker or as an immunocytochemical marker for tissue sections. Serum inhibin levels are of value in the initial diagnosis and subsequent follow-up of sex cord stromal tumors, in particular granulosa cell tumors, and mucinous epithelial adenocarcinomas primarily after menopause. Immunocytochemistry using inhibin alpha subunit antisera can aid in the classification of sex cord stromal tumors and their differentiation from other cancers. Although serum inhibins are elevated in prostatic and testicular cancers, this is of little diagnostic value at the present time.