BACKGROUND: An immune-mediated defect in recognition and elimination of endometrial fragments refluxed in the peritoneal cavity has been hypothesized to play a crucial role in endometriosis development. Since vitamin D is an effective modulator of the immune system, we have hypothesized that the vitamin D status may have a role in the pathogenesis of endometriosis. METHODS: Women of reproductive age selected for surgery for gynecological indications were enrolled in this prospective cohort study. Serum levels of 25-hydroxyvitamin-D 3 , 1,25-dihydroxyvitamin-D 3 and Ca 21 were assessed. RESULTS: Eighty-seven women with endometriosis and 53 controls were recruited. Mean (+ + + + + SD) levels of 25-hydroxyvitamin-D 3 in women with and without endometriosis were 24.9 + + + + + 14.8 ng/ml and 20.4 + + + + + 11.8, respectively (P 5 0.05). The Odds Ratio (95% Confidence Interval) for endometriosis in patients with levels exceeding the 75th percentile of the serum distribution of the molecule (28.2 ng/ml) was 4.8 (1.7-13.5). A positive gradient according to the severity of the disease was also documented. A trend towards higher levels of 1,25-dihydroxyvitamin-D 3 and Ca 21 was observed in women with endometriosis, but differences did not reach statistical significance. As expected, serum concentrations of 25-hydroxyvitamin-D 3 and 1,25-dihydroxyvitamin-D 3, but not Ca 21 , are influenced by the season (P < 0.001, P 5 0.004, P 5 0.57, respectively), while levels of the three molecules did not vary according to the phase of the menstrual cycle. CONCLUSIONS: Endometriosis is associated with higher serum levels of vitamin D.