Osteoprotegerin (OPG) is a soluble receptor for RANKL and therefore a competitive inhibitor of osteoclast differentiation and activity. With this key role in the control of resorptive activity, we found that OPG is a candidate gene for genetic control of bone mass. We examined the promoter and the five exons with surrounding intron sequences of the OPG gene for polymorphisms in 50 normal patients and 50 patients with osteoporosis. We found 12 polymorphisms. Two sets of four and five polymorphisms, respectively, were in complete linkage. Subsequently, we examined the effect of the informative polymorphisms A163-G (promoter), T245-G (promoter), T950-C (promoter), G1181-C (exon 1), and A6890-C (intron 4) on the prevalence of osteoporotic fractures, bone mass, and bone turnover in 268 osteoporotic patients and 327 normal controls. In A163-G the variant allele G was more common among fracture patients: 34.0% versus 26.3% in normal controls (p < 0.05) and the odds ratio (OR) for a vertebral fracture, if an individual has the G allele, was 1.44 (1.00-2.08). In T245-G the variant allele G was more common in osteoporotic patients: 12.4% versus 6.5% (p < 0.02) and the OR for vertebral fracture, if an individual has the G-allele, was 2.00 (1.10-3.62). G1181-C is located in the first exon and causes a shift in the third amino acid from lysine to asparagine. The CC genotype was less common among fracture patients: 26.3% versus 36.7% in the normal controls (p < 0.01). T950-C and A6890-C were not distributed differently among patients with osteoporosis and normal controls. None of the polymorphisms affected bone mineral density (BMD) or biochemical markers of bone turnover in the normal controls. In conclusion, we have examined the human OPG gene for polymorphisms and found 12. The rare alleles of the A163-G and T245-G were significantly more common among patients with vertebral fractures.