This study aims to investigate the (epi)genetic determination of peak bone in men. Two large cohorts: (1) a three-generation family-based case-control study including men with idiopathic osteoporosis and their relatives (n=1123), (2) young healthy men (n=768) in a sib-pair design (295 independent families) at age of peak bone mass will be studied. Genetic polymorphisms in the androgen related genes will be studied in relation to volumetric bone mass, body composition (by DEXA), muscle strenght and hormonal status, together with their interreation. Epigenetic inheritance of bone size and density will be studied using selective breeding in animals.