Background: This study was designed to investigate the effects of phytoestrogen isoflavone on balloon catheter-induced hyperplasia of carotid artery. Methods: Forty-eight female New Zealand rabbits were randomly divided into four groups: control (balloon-induced carotid artery injury only); ovariectomy control (ovariectomy and carotid artery injury), oestrogen (ovariectomy, carotid artery injury and nilestriol, 5. mg/kg daily for 28 days), and isoflavone (ovariectomy, carotid artery injury and isoflavone 120. mg/kg daily for 28 days). The arterial wall thickness was assessed by coloured ultrasonography, and the oestrogen-' and oestrogen-ÃŸ receptors in the abdominal aorta were measured by Western blotting. Results: The medial layer thickness in the isoflavone group was less than in the ovariectomy control group (0.28 ± 0.03 vs. 0.35 ± 0.04. mm, p< 0.01), and the intimal/medial layer (I/. M) ratio is the isoflavone group was also less than in the ovariectomy control group (16.85 ± 3.79 vs. 48.94 ± 8.92, p< 0.01). There was no statistically significant difference in the medial layer thickness or I/. M ratio between the isoflavone and the oestrogen groups. The optical density of the oestrogen-' receptors in the isoflavone group (0.317 ± 0.002) was less than in the oestrogen (0.633 ± 0.002) or ovariectomy control group (0.590 ± 0.001, p< 0.01). The optical density of the oestrogen-ÃŸ receptors in the isoflavone group (1.350 ± 0.002) and the ovariectomy control group (1.2033 ± 0.002) was less than in the oestrogen group (1.7699 ± 0.003, p< 0.01). Conclusions: Isoflavone therapy in the ovariectomised rabbit model attenuated balloon catheter-induced intimal and medial layer hyperplasia in the carotid arteries. Down-regulation of the oestrogen-' receptors may be involved in the hyperplasia-preventative effect.