Tumors derived from a hormonal target organ are assumed to be stimulated by the same hormone that stimulates the normal target tissue. In spite of attempts to acquire direct indications of a correlation between hormones and cancer, none have been definitive because studies of total and free hormone levels have given contradictory results. For this reason, attention has shifted to the study of plasma binding and transport of hormones, that is, of the proteins responsible for modulation of the hormone effect and thus of hormone bioavailability. The data reviewed indicate that in-depth study of the transport and binding system of sex steroids would give new information about the endocrine characteristics of cancer patients.