Concentrations of LH, FSH, oestradiol (E) and progesterone (P) were measured in serum of 12 women with minimal endometriosis and otherwise unexplained infertility. Values were compared with those on corresponding days relative to the pre-ovulatory LH peak (Day 0) in six fertile women. Three women exhibited cycle profiles of LH, FSH, E and P indistinguishable from those in the control group. In the remaining nine women cycle profiles for FSH and follicular phase profiles for LH were normal but eight exhibited a delay in P secretion and reduced total P output. LH concentrations were elevated during the early luteal phase in five subjects, two of whom had a second LH surge. These data suggest that luteal dysfunction and abnormal secretory patterns for LH may be contributory to infertility associated with endometriosis.