A high incidence of restenosis at the site of successful PTCA is a major problem with this procedure. Among the various factors, thromboxane A₂ (TXA₂) and PGI₂ may have important roles on the restenosis in this mechanism. DP-1904 (DP), an imidazole derivative, is a potent selective TXA₂ synthetase inhibitor with prolonged activity. It also increases PGI₂ formation. In this study, the effect of DP on post-PTCA restenosis was compared with a placebo control taken from a recent trial conducted under an equivalent protocol. Angina and OMI patients receiving first elective PICA were enrolled in the study. Oral dosing of DP 600mg/day started at least 3 days before PTCA and continued until re-CAG at 3 months after PICA At re-CAG, restenosis was evaluated by lesion and patient based on the patency achieved at PICA. 1) Very good: no restenosis or expansion, 2) Good: <50% loss, 3) Poor: 50%≤loss<100%, 4) Very poor: 100% loss or more progressed stenosis before PICA. 19 patients (29 lesions) were evaluated in the DP group, and 15 (26 lesions) in the placebo group. DP: placebo evaluation of the stenotic lesion was: Very good 9: 5, Good 15: 10, Poor 2: 8 and Very poor 3: 3, indicating DP-s efficacy (Wilcoxon P=0.10). Evaluation by patient restenosis was: 26.3% in the DP group, 66.7% in the placebo group, showing DP's statistically significant efficacy over placebo (Wilcoxon P<0.05). No side effects were observed in the DP group. Plasma TXB₂/6-keto-PGF_1α ratio showed preferable changes during DP dosing: pre-dosing (2.58±0.82pg/ml), pre-PICA (0.60±0.13), immediately post-PICA (0.57±0.09) and 3 months post-PICA (0.84±0.09).