Cul-de-sac fluid from women with histologically confirmed endometriosis (n = 45) or from infertile women without evidence of endometriosis (n = 28) was collected at the time of laparoscopy during the periovulatory period (days 13 to 18). This fluid was analyzed for prostaglandin E2 (PGE2), prostaglandin F(2a)), (PGF(2a)), 13,14-dihydro-15 keto-PGF(2a) (PGFM), and thromboxane B2 (TXB2) by radioimmunoassay (RIA). Protein content of the fluid also was determined. No difference (P > 0.05) in cul-de-sac fluid volume was found between women with and without endometriosis, nor were differences detected in the level of any of the prostanoids measured in fluid from infertile control patients compared with those with endometriosis. This was true regardless of whether the prostanoids were expressed as a concentration, total amount in fluid, or as a ratio of prostanoid to protein content. The present study does not support the theory that cul-de-sac fluid prostanoids provide a useful diagnostic index of endometriosis.