['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.', '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(2+) were assessed.', '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 = 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(2+) 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(2+), are influenced by the season (P < 0.001, P = 0.004, P = 0.57, respectively), while levels of the three molecules did not vary according to the phase of the menstrual cycle.'] Is vitamin D reserve higher in women with endometriosis?