This study was designed to investigate whether there is any association between body mass index (BMI) and life-style factors, with a special emphasis on calcium and vitamin D intakes. In 1994/1995, men (n = 9252) and women (n = 9662) participated in the fourth Tromsø study and completed the food-frequency and life-style factor questionnaires. We measured BMI, coffee and alcohol consumption, physical activity, smoking, and calcium and vitamin D intakes. A negative association between physical activity, smoking and BMI, and a positive association between BMI and coffee intake were found in both sexes (P < 0.001). BMI and calcium intake were positively related in men (P < 0.001), but not in women. BMI and vitamin D intake were negatively associated in both sexes (P < 0.001), which to our knowledge has not been reported before. The lowest quartile of vitamin D intake was an independent predictor of obesity (defined as BMI >30 kg/m(2)) in men and women (P < 0.001 in both genders), resulting in odds ratios of 2.24 [95% confidence interval (CI) 1.80, 2.80] in men and 1.51 (95% CI 1.23, 1.85) in women compared with the highest quartile. In conclusion, calcium and vitamin D intakes may have opposing effects on body weight, which is difficult to explain given current knowledge of calcium metabolism.