Objective: To study the relationships of molecularly defined lactose malabsorption (LM) and self-reported lactose intolerance (LI) to bone mineral density (BMD) and fractures among Finnish postmenopausal women. Design: A cross-sectional study of two cohorts. Setting: Helsinki University Central Hospital. Subjects: One cohort was population-based and comprised 453 women, aged 62-78 (mean 69) y. Another comprised 52 women, aged 69-85 (mean 75) y, with osteoporotic fractures and 59 control women, aged 69-83 (mean 74) y, without osteoporosis. Methods: A single nucleotide polymorphism of the lactase (LCT) gene at chromosome 2q21-22 was studied. It shows complete association with intestinal disaccharidase activity, with the genotype CC À13 910 meaning adult-type hypolactasia (primary LM) and the genotypes CT À13 910 and TT À13 910 lactose absorption. BMD of the heel was measured by dual-energy X-ray absorptiometry (DXA). Results: In the population-based cohort, 16.0% of women had self-reported LI but only 15.3% of them had the CC À13 910 genotype. Calcium intake from dairy products (P ¼ 0.10) and BMD, adjusted for age, weight, height, exercise, smoking, and estrogen use (P ¼ 0.71) were similar for the genotypes. Women with self-reported LI had reduced calcium intake from dairy products (Po0.0001) but they were more frequent users of calcium supplements than lactose-tolerants (Po0.0001). Adjusted BMD was similar for lactose intolerant and tolerant women (P ¼ 0.60). Of 104 women with previous fracture in the populationbased cohort, 13.5% had the CC À13 910 genotype, which did not differ from the prevalence of 19.3% among 347 women without fractures (P ¼ 0.29). The frequency of the CC À13 910 genotype (23.1%) for 52 women with established osteoporosis was similar as for 59 control women (15.3%) (P ¼ 0.19). Conclusion: Molecularly defined LM and self-reported LI are not risk factors for osteoporosis, if calcium intake from diet and/or supplements remains sufficient. Our study confirms the poor correlation between self-reported LI and LM established by different techniques.