Recent research suggests that 1,25-dihydroxyvitamin D [1,25(OH)(2)D], a steroid hormone that regulates calcium homeostasis, may also play a role in the development and progression of cancer, multiple sclerosis, cardiovascular, and other diseases. Decreased serum 1,25(OH)(2)D concentrations are often observed in overweight and obese patients. However, little is known about the factors that may influence 1,25(OH)(2)D renal synthesis, because it is generally accepted that serum 1,25(OH)(2)D concentration is strictly regulated by parathyroid hormone and serum concentrations of calcium and phosphorus. In this study, the associations among serum 1,25(OH)(2)D, serum 25-hydroxyvitamin D [25(OH)D], and body composition were analyzed in 1779 patients with excess body weight registered in a Metabolic and Medical Lifestyle Management Clinic in Oslo, Norway. According to our results, serum 25(OH)D, adiposity, age, season of blood sampling, and gender directly influence serum 1,25(OH)(2)D (r = 0.33; P < 0.001), with serum 25(OH)D being the strongest predictor for serum 1,25(OH)(2)D. The 1,25(OH)(2)D concentrations were 25.4 pmol/L (95% Cl: 19.3-31.5; P < 0.001) lower in the lowest 25(OH)D quartile to compared with highest quartile. A seasonal variation was observed for both vitamin D metabolites. Thus, our results suggest that in patients with excess body weight, serum 1,25(OH)(2)D concentrations were associated with 25(OH)D and varied during the year. Therefore, it may also be valuable to measure both serum 25(OH)D and 1,25(OH)(2)D for the evaluation of vitamin D status in overweight and obese persons.