A woman presented with progressive hirsutism, deepening of the voice, clitoromegaly and increased libido. The preoperative serum testosterone level was 2,042 ng/dL. The intraoperative ovarian venous blood testosterone levels were 56,327 ng/dL on the left and 1,417 on the right. After a bilateral salpingo-oophorectomy, the serum testosterone level was 20.7 ng/dL. Initial pathologic examination of the ovaries revealed no potential source of testosterone, but reexamination revealed a pure lipoid cell tumor. Intraoperative ovarian vein androgen measurements can be helpful in establishing a histopathologic diagnosis in cases of microscopic virilizing ovarian neoplasms. Ovarian vein sampling for androgen was essential in locating this patient's microscopic tumor.