Genetic factors are major determinants of adult bone density, however, it is unknown how these effects may be mediated. Since bone mineral density is the net result of bone formation and bone resorption we studied biochemical indices of bone formation (serum osteocalcin) and resorption [fasting urinary calcium:creatinine (Ca/Crt) and hydroxyproline:creatinine (OH/Crt)] in adult female twins; 39 monozygotic (MZ) and 31 dizygotic (DZ) twin pairs (age, mean +/- SEM, MZ: 51.1 +/- 1.5 yrs; DZ: 46.5 +/- 1.5 yrs, P = NS). Of these subjects, 18 MZ twin pairs and 10 DZ twin pairs were postmenopausal. The MZ twin pair correlations (rMZ) for each index of bone turnover exceeded that between DZ pairs (rDZ), but this difference was only significant for osteocalcin (rMZ = 0.81, rDZ = 0.21, P less than 0.001). Similarly, in the postmenopausal group examined alone, the rMZ (r = 0.84) for serum osteocalcin was significantly greater than rDZ (r = -0.003, P less than 0.03). These osteocalcin data imply that 80% of the variance in serum osteocalcin could be explained by genetic factors. Although genetic effects on fasting urinary hydroxyproline:creatine and calcium:creatinine were not demonstrable, these indices may be less precise and specific. The data indicate that circulating osteocalcin, and therefore bone formation, is strongly genetically determined. These studies suggest at least one of the mechanisms of the genetic effect on bone mass relates to the regulation of bone turnover.