OBJECTIVES Animal models have described critical roles for transforming growth factor beta (TGF beta) isoforms in modulating urinary tract stroma phenotype. TGF beta 3 is of particular interest because it may regulate TGF beta 1 and TGF beta 2 expression, but its modulatory affect has not been so well characterized in human cells. In this study, we aim to determine whether TGF beta 3 treatment induced differentiation of human urinary tract stroma-derived fibroblasts to a smooth muscle-like phenotype. METHODS We established cultures of human urinary tract stroma-derived fibroblasts and studied the effects of TGF beta 3 treatment using proliferation assays, cell cycle analysis, immunocytochemistry, and Western blotting for expression of differentiation marker and downstream regulators, and fura-2 fluorescence to study the effects on intracellular calcium. RESULTS TGF beta 3 treatment induced proliferation that peaked at 72 hours, followed by enhanced expression of alpha-smooth muscle actin (alpha-SMA) with a maximal 3.4-fold increase at 168 hours. TGF beta 3 treatment decreased resting [Ca2+](i) by 70% and caused a 95% decrease in stimulated internal Ca2+ release regulated by the sarcoplasmic/endoplasmic calcium-ATPase pump. These effects were associated with upregulation of nuclear activator of T cells -1 (NFAT), a known regulator of cell differentiation. CONCLUSIONS TGF beta 3 treatment causes a time-specific response in the presence of serum, whereby fibroblasts initially proliferate and subsequently differentiate to a smooth muscle-like phenotype. This sequence was associated with stabilization of [Ca2+](i) stores, suggesting a role in the induction of hyperplasia and reduction of contractility; phenomena associated with a number of urinary tract pathologies.