To assess the influence of collaterals on the long-term follow-up after successful percutaneous transluminal coronary arteriography (PTCA), 120 consecutive patients were studied. Of these, 104 (87%) had an adequate reangiogram and were included. At the time of PTCA the collaterals were estimated by a scoring system. In addition, the coronary wedge pressure was measured in 49 patients six months after PTCA, a follow-up angiogram was performed, and the patients were split up into a group with restenoses (stenosis greater than 50%), 34 patients (32.7%); and a group without restenoses (stenosis less than 50%), 70 patients (67.3%). A total of 35 patients (30.7%) had collaterals. A comparison between both groups showed no significant differences in clinical parameters (age, angina duration, vessel involvement, lipids, blood sugar, and blood pressure), and stenoses-related parameters (degree of stenoses, eccentricity, balloon size, inflation pressure, dissection, gradient after dilatation, and residual stenoses). Patients with collaterals had a significantly higher incidence of restenoses than those without collaterals (45.7% vs. 26.1%, p less than 0.05). Patients with wedge pressure of less than 45 mmHg (n = 30) had a significantly lower restenosis rate (23.3%) than patients with a coronary wedge pressure of greater than 45 mmHg (n = 19) (restenosis rate 52.6%). It is concluded that the presence of collaterals indicates a high restenosis rate after PTCA within 6 months.