Serum and peritoneal fluid concentrations of progesterone and 17 beta-oestradiol were measured during days 20-27 of the luteal phase in 33 women with minimal stage endometriosis and 21 with unexplained infertility. The results were analysed using Student's t-test and straight line regression analysis. In both groups, 17 beta-oestradiol and progesterone values were several-fold higher than the corresponding serum values and there were significant correlations between the peritoneal fluid and the serum steroid values. In the peritoneal fluid there was a significant reduction in both progesterone and its ratio with 17 beta-oestradiol in women with minimal stage endometriosis (P < 0.03, P < 0.04, respectively). The peritoneal fluid levels of 17 beta-oestradiol in women with minimal stage endometriosis were, however, not significantly altered. Despite the correlations between peritoneal fluid and serum steroid levels, there were no significant changes in the serum concentrations of progesterone, 17 beta-oestradiol or the ratio of progesterone to 17 beta-oestradiol in women with minimal stage endometriosis. It is concluded that luteal phase progesterone and its relationship to 17 beta-oestradiol are subtly altered in infertile women with minimal stage endometriosis compared with women with unexplained infertility. These alterations may be more clearly demonstrated in peritoneal fluid rather than in peripheral blood, possibly because the latter contains steroids which are of non-ovarian origin and are also constantly affected by the liver enzyme activity and enterohepatic circulation of individual subjects.