Diabetes mellitus (DM) is an established risk factor for stent restenosis, in part as a result of the smaller vessel dimensions and longer lesions. This study compared the magnitude of acute lumen gain and late lumen loss after elective coronary stent implantation in patients with and without DM using a matched-pair analysis. A total of 133 patients with DM and 192 coronary lesions were included in this analysis. A group of 192 lesions in 182 patients without DM were matched in a pairwise fashion, stratifying for reference diameter, minimal luminal diameter, and lesion length. The binary restenosis rate at the 5-month follow-up angiography was 25% in the DM group and 14% in the non-DM group (p <0.01). Acute angiographic lumen gain (1.47 ± 0.41 vs 1.56 ± 0.38 mm, p = 0.03) and late lumen loss (0.64 ± 0.42 vs 0.55 ± 0.36 mm, p = 0.02) were significantly different between the DM and non-DM groups. In conclusion, suboptimal acute procedural results and an exaggerated neointimal proliferation contributed by about 50% to the lower net lumen gain in the DM group. Patients with DM had a significantly higher restenosis rate even when matched for preprocedural angiographic lesion dimensions. Mechanistically, inferior procedural results, as well as exaggerated neointimal proliferation, are, quantitatively, equally important in this process.