Osteoblasts and adipocytes are derived from common bone marrow stromal cells that play crucial roles in the generation of osteoclasts. Activation of peroxisome proliferator-activated receptor-gamma (PPARgamma) induces adipogenic differentiation of stromal cells; however, whether this would affect osteoblast/osteoclast differentiation is unknown. Thus, we examined the effects of the thiazolidinedione (TZD) class of antidiabetic agents that activate PPARgamma on osteoblast/osteoclast differentiation using mouse whole bone marrow cell culture. As reported, all TZDs we tested (troglitazone, pioglitazone, and BRL 49653) markedly increased the number of Oil Red O-positive adipocytes and the expression of adipsin and PPARgamma 2. 1alpha,25-Dihydroxyvitamin D3 [1,25-(OH)2D3] did not affect adipogenic differentiation induced by TZDs. TZDs did not affect alkaline phosphatase activity, an early marker of osteoblastic differentiation, despite their marked adipogenic effects. TZDs decreased the number of tartrate-resistant acid phosphatase-positive multinucleated osteoclast-like cells induced by 1,25-(OH)2D3 or PTH. Troglitazone dose dependently inhibited basal and 1,25-(OH)2D3- and PTH-induced bone resorption as assessed by pit formation assay. Interleukin-11 blocked the induction by troglitazone of adipogenesis, but had no effect on the inhibition of osteoclast-like cell formation. These results indicate that TZDs are potent inhibitors of bone resorption in vitro. Inhibitory effects of TZDs on osteoclastic bone resorption was not osteotropic factor specific and did not appear to be related to their adipogenic effects. Thus, TZDs may suppress bone resorption in diabetic patients and prevent bone loss.