Osteoporosis is a major public health problem characterized by low bone mineral density (BMD) that presently has no biochemical test useful for its diagnosis. The cytokine TGF-beta has been postulated to play a role in controlling bone density by regulating the fine balance between bone matrix deposition by osteoblasts and its resorption by osteoclasts. We explored whether measurement of serum levels of different TGF-beta isoforms could be useful as a clinical tool in osteoporosis. We measured the concentration of TGF-beta1 antigen using the BDA19 capture sandwich enzyme-linked immunosorbent assay (ELISA), TGF-beta2 antigen concentration using a Quantikine sandwich ELISA kit and TGF-beta3 antigen concentration using a modified version of the TGF-beta1 Quantikine sandwich ELISA kit. Subjects were 41 women with osteoporosis (with nontraumatic vertebral fracture or lumbar spine BMD Z-score <-1.5 SD) and a total of 199 control women from different sources. Serum concentrations of TGF-beta1 and TGF-beta2 were similar in all groups. However, detectable levels of TGF-beta3 (>0.2 ng/ml) were found in 35 of 41 patients with osteoporosis (median 7.2 (5.2-8.9) ng/ml) compared with 11 of 36 controls or 24 of 89 healthy women of unknown bone density. Differences among the groups could not be accounted for by age, weight, medications, use of hormone replacement therapy or the presence of osteoarthritis. Using the optimal cut-off of >/=2 ng/ml, the test was able to detect an individual with low spine BMD (Z-score <-1.5) with a sensitivity of 84% and a specificity of 53%, with similar results for the femoral neck. The odds ratio for osteoporosis associated with a positive test at this level was 5.93 (95% CI 2.41-11.59), and 4.1 (95% CI 1.66-10.11) using the WHO cut-off of T-score <-2.5. Serum TGF-beta3 concentration is raised in osteoporotic women and the test appears to have potential as a marker for osteoporosis. The underlying mechanisms and the relationships between TGF-beta3 and bone turnover and fractures remain to be explored.