From among a group of women with apparently normal menstrual cycles, 7 cycles with short luteal phases were identified. These cycles were characterized by grossly normal FSH and LH patterns although the FSH/LH ratio was below normal. Mean plasma progesterone increased to less than 2 ng/ml shortly after the LH peak; thus, the peak level was lower and the peak occurred earlier than in the normal cycle. Plasma 17-hydroxyprogesterone increased at the time of the LH peak although the mean peak level was only 60% that of normal cycles. There was no secondary increase of 17-hydroxyprogesterone during the luteal phase. These observations are consistent with the hypothesis that a relative deficiency of FSH during the follicular phase results in diminished follicular development and subsequent inadequate corpus luteum formation or function.