Steroid synthesis in luteal/granulosa cells harvested after follicular aspiration in a patient with 17α-hydroxylase deficiency was investigated. Follicular stimulation with purified FSH after the suppression of ACTH and gonadotropin secretion by corticoid analogs and superactive GnRH agonist permitted ovum pick-up and the study of steroid synthesis in the obtained granulosa cells. Progesterone synthesis was elevated while 17α-hydroxy progesterone, testosterone, androstenedione, and estrogen production was minimal. Aromatase activity was retained in these cells, as demonstrated by the 100-fold increase in estrogen levels after the addition of androgens. Progesterone levels in follicular fluids were in the normal range, while estrogen, 17α-hydroxy progesterone, and androgen levels were minimal. These results provide in vitro evidence of 17α-hydroxylase deficiency in the granulosa cells of this patient.