Siah1a has been implicated in numerous signaling pathways because of its ability to induce ubiquitin-mediated degradation of many protein substrates. Siah1a knockout mice are growth-retarded, exhibit early lethality, and display spermatogenic defects. In this study we identified a striking low bone volume phenotype in these mice (trabecular bone volume was halved compared with wild type mice), linking Siah1a to bone metabolism for the first time. Markers of bone formation, including osteoblast numbers and osteoid volume, were decreased by up to 40%, whereas the number of osteoclasts was more than doubled in Siah1a mutant mice. However, ex vivo osteoclast formation occurs normally and hematopoietic osteoclast progenitor cell types were present in normal numbers in Siah1a mutant mice. Moreover, adoptive transfer of Siah1a mutant bone marrow into wild type mice failed to reproduce the osteopenia or increased osteoclast numbers observed in mutant mice. Although ex vivo osteoblast colony formation was normal in Siah1a mutant mice, mineralization from these cells was elevated in cultures from Siah1a mutant mice, which may explain the reduction in osteoid volume seen in vivo. These findings suggest that although Siah1a is clearly essential for normal bone metabolism, the bone defect in Siah1a mutant mice is not due to cell-autonomous requirements for Siah1a in osteoblast or osteoclast formation. We propose that bone metabolism defects in Siah1a mutant mice are secondary to an alteration in an unidentified systemic, paracrine, or metabolic factor in these mice.