Coronary in-stent restenosis might be triggered by contact allergy to nickel, chromate, or molybdenum ions released from stainless-steel stents. We investigated the association between allergic reactions to stent components and the occurrence of in-stent restenosis. Patients with coronary stainless-steel stents who underwent angiography for suspected restenosis were consecutively included in this study. Quantitative coronary angiography for analysis of percentage diameter stenosis was done on 131 patients (mean age 62 years [SD 9]) with 171 stents 6.1 months (2.7) after stent implantation. All patients underwent epicutaneous patch tests (Finn chamber method) for nickel, chromate, molybdenum, manganese, and small 316L stainless-steel plates. Patch tests were assessed by independent dermatologists after 48 h, 72 h, and when necessary 96 h of contact with the potential allergen. In-stent restenosis (> or =550% diameter stenosis) occurred in 89 patients. All ten patients with positive patch-test results had restenoses (p=0.03). Four male patients had positive reactions to molybdenum, and seven patients (four male, three female) had reactions to nickel. No patient with an allergic reaction to the standard test substances had a positive reaction to the stainless-steel plates. All patients with positive results had recurrent angina pectoris and needed target-vessel revascularisation. Patients with allergic patch-test reactions to nickel and molybdenum had a higher frequency of in-stent restenoses than patients without hypersensitivity. Allergic reactions to nickel and molybdenum released from stents may be one of the triggering mechanisms for in-stent restenosis.