Objectives: Most investigators agree that endometriosis is associated with a state of subclinical, non-infectious peritoneal inflammation. The objective of the study was to assess concentrations of two markers of the acute inflammatory phase proteins, haptoglobin and ceruloplasmin, in peritoneal fluid of endometriotic women. Material and methods: 229 women who underwent diagnostic or therapeutic laparoscopy were included in the study. Minimal, mild, moderate and severe endometriosis according to ASRM was confirmed in 119 women (study groups), whereas 110 patients suffered from simple serous or dermoid ovarian cysts (reference groups). Haptoglobin and ceruloplasmin concentrations in the peritoneal fluid samples aspirated during laparoscopy were measured using commercially available radial immunodiffusion kits. Results: The concentration of haptoglobin in the peritoneal fluid of women with endometriosis was significantly higher as compared to patients with serous and dermoid ovarian cysts. Significantly higher haptoglobin level was observed in patients with severe and moderate endometriosis as compared to women from both reference groups. No significant difference in the peritoneal fluid ceruloplasmin levels was found between patients with endometriosis and women from reference groups. However, it was noted that ceruloplasmin levels are higher in the subgroup of patients with severe endometriosis as compared to both reference groups and women with mild disease. Conclusions: Our results support the hypothesis that endometriosis is associated with subclinical inflammation within the peritoneal cavity. It may be speculated that pro-inflammatory stimuli strong enough to cause an increase in acute inflammatory phase proteins peritoneal fluid concentrations are observed only in the advanced stages of the disease.