Background: Malabsorption of vitamin D and calcium is a known side effect of laparoscopic gastric bypass (LGB). Our objective was to identify the prevalence of abnormalities in calcium, parathormone (PTH), and 25-OH vitamin D levels in patients undergoing LGB, and the correlation of reported compliance with supplementation, and seasonal variation on vitamin D levels. Methods: Patients undergoing LGB from September 2003 through June 2004 were included. Calcium and PTH levels were obtained preoperatively. At 1 year postoperatively, calcium, PTH, and vitamin D levels were obtained. Laboratory values were classified as low, normal, or high, based on standard reference ranges. Results were analyzed using the exact binomial test. Results: One-year follow-up laboratory data were available for 70 patients: 10 males and 60 females with a mean age of 44.7 years. The mean preoperative body mass index was 47.4 kg/m 2. Mean percent excess weight loss at 1 year was 73.4. Nine patients (14.3) had elevated PTH levels preoperatively, and seven patients (10.3) had elevated PTH postoperatively (p ≤ 0.001). The incidence of low 25-OH vitamin D levels postoperatively was 40.4. Conclusions: A significant percentage of patients undergoing LGB present with an abnormally high PTH level. Abnormally low vitamin D levels are quite prevalent 1 year after LGB, despite normal serum calcium levels. The month of surgery (summer vs. winter) did not have an impact on PTH, calcium, or vitamin D levels. Patients' self-reported compliance or noncompliance with recommended supplementation did not correlate with their vitamin D levels.