Serum levels of sex hormone binding globulin (SHBG), total testosterone (T), free testosterone (fT), 4-androstene-3,17-dione (A-4) and dehydroepiandrosterone sulfate (DHAS) were determined in 30 patients with the polycystic ovary syndrome. Subnormal levels of SHBG were found in 60% of the patients. Elevated levels of T were found in 53%, of fT in 53%, of the T/SHBG ratio in 90%, of A-4 in 75% and of DHAS in 36% of the patients. The better diagnostic accuracy of the T/SHBG ratio compared to fT may be explained by the regulatory effect of SHBG binding upon the albumin-bound fraction of T, which may be another biologically active T fraction. It is concluded that the assay of fT does not offer any further diagnostic advance compared to conventional determinations of T. Assays of fT do not provide any information about the origin of the elevated levels of biologically active androgen.