Background Vascular smooth muscle cells (VSMCs) can express heme-oxygenase (HO), a rate-limiting enzyme in the degradation of heme to bilirubin, ferritin and carbon monoxide (CO). VSMC-derived CO can suppress VSMC proliferation and 05-serve as an antiproliferation factor. The promoter region of HO-1 shows a polymorphism with different (GT)n repeats that has been reported to differently induce gene expression. The objective of this study was to examine the effect of this variation on the occurrence of restenosis after in-stent treatment in patients with coronary artery disease. Methods Candidates who underwent coronary stent implantation were genotyped for the HO-1 promoter polymorphism using polymerase chain reaction (PCR) and automated DNA capillary sequencer. Serum levels of IL-6 and C-reactive protein (CRP) were obtained at baseline, 24 hours and 48 hours after stenting. The primary end point for the study was angiographic evidence of in-stent restenosis at 6 months. All parameters for evaluation of restenosis were analysed by quantitatve computer-assisted angiographic analysis (QCA). Results One hundred and eighty-seven patients who underwent coronary stent implantation were studied of whom 27.8% showed ≥50% restenosis after 6 months. The distribution of (GT)n repeats of all patients in the promoter region of HO-1 genotype ranged from 22 to 42, with (GT)25 and (GT)32 being the two most common alleles. The allelic repeats were divided into the short class (S) with 29 (GT)n, the middle class (M) with 30-37 (GT)n and the long class (L) with 38 (GT)n. There was no significant difference in the restenosis between the genotype groups or between post operation levels of inflammation markers, but carriers of the S allele (n=120) had 33.3% lower baseline IL-6 compared with non-S carriers (n=67, P=0.0008). Conclusions Although no association was observed between the HO-1 promoter polymorphism and coronary in-stent restenosis following the stent procedure, the association with plasma IL-6 levels suggests that HO-1 S allele might protect from the atherosclerotic inflammatory process.