It is generally accepted that the current scoring system for endometriosis has little correlation with clinical symptoms such as pain, and therefore we may deduce that either endometriosis does not cause pain, or that the current scoring system does not indicate the biological activity of the disease. Pain may occur because the presence of endometriosis produces an intraperitoneal inflammatory response, and several studies have shown that the cytokine content of peritoneal fluid differs between women with and without endometriosis. We studied the relationship between tumour necrosis factor alpha (TNF alpha), platelet-derived growth factor (PDGF), interleukin (IL)-6, IL-4 and TNF (alpha and beta) activity in peritoneal fluid and the clinical history of pain and infertility. TNF alpha concentrations were increased in peritoneal fluid of women with endometriosis and of infertile women; PDGF concentrations were increased in peritoneal fluid of parous women; IL-6 was increased in peritoneal fluid of women with adhesions; IL-4 was absent from peritoneal fluid. PDGF and IL-6 concentrations were cycle related, with the highest amounts in the menstrual and proliferative phases respectively. We failed to demonstrate any association between concentrations of cytokines in vitro and pain symptoms or severity of endometriosis.