We performed a retrospective study of 237 patients attending a specialty osteoporosis practice. Secondary causes for reduced bone mineral density (BMD) were evaluated in 196 postmenopausal women and 41 premenopausal women; mean age was 56 +/- 13.8 years (mean +/- SD). BMD was measured by dual-energy X-ray absorptiometry (DXA) (QDR 1000W/2000 Hologic). Levels of intact parathyroid hormone (iPTH), calcidiol [25(OH)D], thyroid-stimulating hormone, and 24-hour urinary calcium were measured, and serum and urine protein (SPEP and UPEP) electrophoresis were performed. Overall, 16% of our patients had 25(OH)D levels <15 ng/ml, the lowest acceptable vitamin D level without a concomitant rise in iPTH levels. Among the osteoporotic patients (T score <-2.5 SD), 17% had 25(OH)D levels <15 ng/ml and 7% <10 ng/ml. Among the osteopenic patients (-2.5 < T < -1.0 SD), 11% had 25(OH)D levels <15 ng/ml. Seventeen percent of patients with Z score </=-1.0 SD (low range normal value) had 25(OH)D levels <15 ng/ml. Low 25(OH)D levels were inversely related to high iPTH values (r = 0.30, P < 0.0001). Hypercalciuria was present in 15% of our patients, elevations of PTH levels (>65 pg/ml, upper normal limit of assay) were present in 11.5%, and hyperthyroidism in 4%. A 25(OH)D level of <25 ng/ml in women (n = 86) with no known secondary causes of low BMD was associated with an iPTH level above 49 pg/ml. The measurement of 25(OH)D levels is recommended in the evaluation of secondary causes for reduced BMD. Supplementation with vitamin D appears needed to keep 25(OH)D above 25 ng/ml, the level required to prevent increments in iPTH levels.