BACKGROUND: In vitro experimental studies have reported that transforming growth factor-beta1 (TGF-beta1) stimulates the production of alpha-smooth muscle actin (alpha-SMA) in porcine valves. However, the relation between TGF-beta1 and alpha-SMA in myxomatous valves has not been elucidated. METHODS: We classified 27 subjects into two groups: 1) myxomatous group (M:F=11:12, mean age=55+/-15 years) and 2) rheumatic group (M:F=3:1, mean age=41+/-17 years) according to preoperative echocardiographic and postoperative histologic findings. Twenty-seven valve specimens from the patients who underwent valve replacement were obtained. Tissue samples were analyzed by immunohistochemistry for TGF-beta1 and alpha-SMA. The positively stained areas were measured using an image analysis program (Image Pro-Plus 4.5), and then the TGF-beta1 volume fraction (TGF-VF) and alpha-SMA volume fraction (alpha-SMA-VF) were calculated. RESULTS: TGF-VF in myxomatous valves was higher than in rheumatic valves (2,759+/-2,294 vs 864+/-276, p=0.04). alpha-SMA-VF in myxomatous valves was higher than in rheumatic valves (4,122+/-2,275 vs 2,421+/-844, p=0.002). There was a significant correlation between TGF-beta1 and alpha-SMA in myxomatous valves (r=0.38, p=0.04). There was no significant correlation between TGF-beta1 and alpha-SMA in rheumatic valves (r=-0.50, p=0.67). CONCLUSIONS: TGF-beta1 and alpha-SMA may be related to the pathogenesis of myxomatous valves. The activation of TGF-beta1 might increase the expression of alpha-SMA in human myxomatous valves.