Normal probability plots were used to assess the homogeneity of a population of 327 luteal phases from apparently ovulatory menstrual cycles. The length of the luteal phase was defined as the interval (in days) following but not including, the luteinizing hormone peak, up to and including the day before onset of menstruation. A small sub-set of the population consisted of cycles with abnormally short luteal phases but the majority of the data followed a normal frequency distribution which gave a mean (+/- SD) for normal luteal phase length of 14.13 (+/- 1.41) days. It was estimated that all cycles with a luteal phase less than or equal to 9 days were abnormal, and that 74%, 22% and 2% respectively of cycles with luteal phases of 10, 11 and 12 days were also abnormal. The total incidence of short luteal phases defined as above was 5.2%.