Aim To study the influence of hirudin on thrombin-induced proliferation of vascular smooth muscle cells (VSMC) and its regulated mechanism in vitro. Methods VSMCs were isolated from the thoracic aorta of rabbits and cultured 2 - 5 passages were incubated with thrombin (4.0 ku/L), while hirudin (6.0 ku/L) and heparin (6.0 ku/L) were added except for control group. The effect of hirudin on growth of VSMC was determined by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltet-razohum bromide (MTT) assay and flow cytometry. The expression of platelet-drived growth factor (PDGF) and proliferating cell nuclear antigen (PCNA) in VSMCs was evaluated by immunohistochemistry technique and computer image analysis system. Results Hirudin remarkedly inhibited both the thrombin-induced proliferation and the expression of PDGF and PCNA in VSMC (P< 0.05, P< 0.01). Conclusion Hirudin is able to significantly influence the proliferative activity of VSMCs, whichmay be carried out through inhibition of PDGF and PCNA expression.