Expression of Smad and its signalling cascade in osteosarcoma. Smad signalling is important for inducing the expression of cell cycle inhibitors such as p21/WAF1, thereby facilitating transforming growth factor (TGF)-beta mediated inhibition of cell growth. The role and clinical implications of Smad signalling in osteosarcoma remain unclear.We assessed the immunohistochemical expression profiles of Smad2, P-Smad2, Smad4, and p21/WAF1 proteins in 34 cases of osteosarcoma. We also investigated whether the expression of these proteins was correlated with clinicopathological parameters.The p21/WAF1 expression showed a significant relationship with tumour extension (p = 0.018). Smad4 expression was significantly correlated with Smad2 (p = 0.019) and P-Smad2 (p = 0.006) expression in osteosarcoma. Kaplan-Meier analysis showed that tumour extension (p = 0.0038), TNM stage (p = 0.0096), and p21/WAF1 expression (p = 0.0023) were significantly correlated with overall survival. Cox multivariate analysis showed that tumour extension (p = 0.048) and p21/WAF1 expression (p = 0.022) were independent poor prognostic factors.Smad signalling in osteosarcoma is partially operative. The high p21/WAF1 expression in osteosarcoma is significantly correlated with tumour extension. Our results suggest that overexpression of p21/WAF1 in osteosarcoma might be one of the poor prognostic factors.