Background: Microalbuminuria (MA) is a strong risk factor for subsequent chronic disease, both renal and coronary heart disease. We investigated whether MA could predict in-stent restenosis (ISR) in non-diabetic patients with unstable angina pectoris (UAP). Methods: A total of 221 patients (59.6짹9.4 years, male 56.1%) with UAP who underwent percutaneous coronary intervention (PCI) and follow-up (F-U) coronary angiography were enrolled. We excluded the patients with history of diabetes mellitus, medication of oral hypoglycemics, or hemoglobin A1c (HbA1c) more than 7%. The patients were divided into two groups according to the presence of ISR in F-U CAG: ISR group (n=61) and No ISR group (n=160). Results: Baseline clinical characteristics including the rate of drug-eluting stent, location of target vessel, and inflammatory markers between the groups were similar. F-U monocyte count and apolipoprotein B/apolipoprotein A1 ratio were significantly higher in ISR group (524.9짹208.4 vs. 464.7짹158.1/mm3, p=0.044; 0.6짹0.2 vs. 0.5짹0.2, p=0.027). The baseline MA and urinary albumin excretion (UAE) levels were significantly lower in ISR group than those in no ISR group (10.6짹6.6 vs. 15.9짹22.7 ug/mL, p=0.028; 10.6짹7.4 vs. 17.2짹29.2 mg/gCr, p=0.046). However, F-U MA and UAE levels were significantly higher in ISR group than that in No ISR group (29.5짹35.9 vs. 15.3짹17.1 ug/mL, p=0.039; 38.1짹33.9 vs. 17.1짹25.2 mg/gCr, p=0.020). The change of MA level (F-U MA minus baseline MA, ∆MA) was significantly higher in ISR group (34.1짹36.1 vs. -3.9짹21.9 ug/mL, p=0.044). F-U HbA1c and creatinine levels were within the normal ranges and similar between the groups. ∆MA correlated with F-U monocyte count (r=0.243, p=0.028), F-U C-reactive protein level (r=0.476, p<0.001). In multivariate analysis, elevated ∆MA showed high probability of ISR (odd ratio, 3.21; 95% CI, 1.01 to 10.21, p<0.048). Conclusions: Progressively increased microalbuminuria could be a predictor of ISR in non-diabetic patients with unstable angin pectoris.