The irregular menses experienced by obese and/or hirsute women were studied by daily measurement of serum LH, FSH, estradiol (E2) and progesterone (P). During 8 cycles of 24 to 157 days, each episode of menstrual bleeding was preceded by hormonal evidence of follicular maturation and corpus luteum formation. The premenstrual fluctuations of LH and E2 were not different from those observed in normal cycles. Although the luteal phase was of more than 10 days' duration, progesterone secretion was subnormal during each cycle. Serum FSH was low throughout the time preceding the LH/FSH surge. These data suggest that inadequate corpus luteum progesterone secretion may be characteristic of the menstrual cycles in oligomenorrheic, obese infertile women.