OBJECTIVE: Bone mineral density (BMD) is under strong genetic control and a number of candidate genes have been associated with BMD. Both muscle strength and body weight are considered to be important predictors of BMD but far less is known about the genes affecting muscle strength and fat mass. The purpose of this study was to investigate the poly adenosine (A) repeat and the BsmI SNP in the vitamin D receptor (VDR) in relation to muscle strength and body composition in healthy women. DESIGN: A population-based study of 175 healthy women aged 20-39 years was used. METHODS: The polymorphic regions in the VDR gene (the poly A repeat and the BsmI SNP) were amplified by PCR. Body mass measurements (fat mass, lean mass, body weight and body mass index) and muscle strength (quadriceps, hamstring and grip strength) were evaluated. RESULTS: Individuals with shorter poly A repeat, ss and/or absence of the linked BsmI restriction site (BB) have higher hamstring strength (ss vs LL, P=0.02), body weight (ss vs LL, P=0.049) and fat mass (ss vs LL, P=0.04) compared with women with a longer poly A repeat (LL) and/or the presence of the linked BsmI restriction site (bb). CONCLUSIONS: Genetic variation in the VDR is correlated with muscle strength, fat mass and body weight in premenopausal women. Further functional studies on the poly A microsatellite are needed to elucidate whether this is the functionally relevant locus or if the polymorphism is in linkage disequilibrium with a functional variant in a closely situated gene further downstream of the VDR 3'UTR.