This protocol was performed to elucidate the preventive effects of probucol on restenosis after percutaneous transluminal coronary angioplasty (PTCA). A total of 118 patients with 134 vessels undergoing successful PTCA was randomly and prospectively assigned to the probucol group (group P) or the control group (group C). The subjects consisted of 91 men and 27 women, with a mean age of 63.4 ± 2.3 years. Sixty-six vessels of 59 patients in group P and 68 vessels of 59 patients in group C were evaluated by coronary angiography at 3 months after PTCA. Probucol (0.5 mg/day) was administered between >7 days before PTCA and 3 months after PTCA. The serum total cholesterol (TC) level and the formula low-density lipoprotein cholesterol (formula LDL-C) in group P decreased from 203.8 ± 43.1 to 169.6 ± 39.4 mg/dl and from 131.4 ± 0.7 to 108.7 ± 2.5 mg/dl, whereas in group C, the levels decreased only from 202.3 ± 32.1 to 194.2 ± 29.8 mg/dl and from 129.2 ± 38.1 to 124.3 ± 31.7 mg/dl, respectively. The restenosis rate was significantly lower in group P (19.7%; 13 of 66 vessels than in group C (39.7%; 27 of 68 vessels; p < 0.05) In group P, the probucol blood concentration was significantly higher in the subjects without restenosis (31 ± 9 μg/ml) than in those with restenosis (18 ± 8 μg/ml; p < 0.01), but the serum TC and formula LDL-C levels were not significantly different between these two groups. In summary, long-term administration of probucol significantly reduces the incidence of restenosis after PTCA. It was suggested that the mechanism of this preventive effect was not reducing the serum TC or formula LDL-C levels, but rather an inhibitory action on smooth muscle cell proliferation.