The aim of this study was to determine the influence of peritoneal fluid from patients with minimal stage or treated endometriosis on sperm motility parameters. Peritoneal fluid aspirated at diagnostic laparoscopy for unexplained infertility from women during the luteal phase of the menstrual cycle (days 20-23) was incubated for 5 h with fresh semen samples obtained from men of recently proven fertility. Spermatozoa were prepared by a swim-up technique from unprocessed semen. Using computer-assisted semen analysis (Hamilton-Thorn Research, MA, USA), sperm motility and motion parameters were observed at 0, 120, 180 and 300 min. Compared with spermatozoa incubated in Earle's balanced salt solution/human serum albumin, the percentage motility, percentage progressive motility and progressive velocity of spermatozoa incubated in peritoneal fluid from patients without visible endometriosis were significantly higher (P < 0.05). Maximal effect was observed at 3 h and maintained until 5 h. We conclude that in an in-vitro study, in contrast to peritoneal fluid from patients with minimal stage endometriosis, peritoneal fluid from patients with unexplained infertility and no visible endometriosis can improve sperm motility when compared with culture medium.