Over the past 30 years, numerous studies in invertebrates and vertebrates have established a role of calcium in oocyte maturation as well as in the resumption and progression of follicular development. Polycystic ovarian syndrome (PCO) is characterized by hyperandrogenic chronic anovulation, theca cell hyperplasia, and arrested follicular development. The aim of this observational study was to determine whether vitamin D and calcium dysregulation contribute to the development of follicular arrest in women with PCO, resulting in reproductive and menstrual dysfunction. Thirteen premenopausal women (mean age 31 +/- 7.9 years) with documented chronic anovulation and hyperandrogenism were evaluated. Four women were amenorrheic and nine had a history oligomenorrhea, two of whom had dysfunctional bleeding. Nine had abnormal pelvic sonograms with multiple ovarian follicular cysts. All were hirsute, two had alopecia, and five had acanthosis nigricans. The mean 25 hydrovitamin D was 11.2 +/- 6.9 ng/ml [normal (nl): 9-52], and the mean 1,25 dihydroxyvitamin D was 45.8 +/- 18 pg/ml. with one woman with a 1,25 dihydroxyvitamin D <5 pg/ml (nl: 15-60). The mean intact parathyroid hormone level was 47 +/- 19 pg/ml (nl: 10-65), with five women with abnormally elevated parathyroid hormone levels. All were normocalcemic (9.3 +/- 0.4 mg/dl). Vitamin D repletion with calcium therapy resulted in normalized menstrual cycles within 2 months for seven women, with two experiencing resolution of their dysfunctional bleeding. Two became pregnant, and the other four patients maintained normal menstrual cycles. These data suggest that abnormalities in calcium homeostasis may be responsible, in part, for the arrested follicular development in women with PCO and may contribute to the pathogenesis of PCO.