Increased mean platelet volume is a central process in the pathophysiology of coronary heart disease. Insulin resistance contributes to increased platelet activation.To assess the mean platelet volume and its possible relationship with insulin resistance in non-diabetic patients with slow coronary flow.The study included 60 patients with slow coronary flow and 20 subjects (controls) with normal coronary arteries. Slow coronary flow patients were divided into 2 groups, insulin resistant (32 patients) and insulin sensitive (28 patients) according to the homeostasis model assessment of insulin resistance index (HOMA-IR).Patients with slow coronary flow had significantly higher mean platelet volume values (7.9±0.47 vs. 7.1±0.5, p<0.01), insulin level (10.8±3.2 vs. 8.2±1.4, p<0.01), and HOMA-IR scores (2.72±0.85 vs. 1.84±0.19, p<0.01). These parameters were significantly higher in insulin-resistant patients than in insulin-sensitive ones. The mean platelet volume was correlated with HOMA-IR (r=0.52, p<0.01) and insulin level (r=0.58, p<0.01). In multivariate analysis, mean platelet volume and HOMA-IR were independent predictors of mean TIMI frame count {(B±SE=0.562±2.95, p<0.01) and (B±SE=0.538±2.46, p<0.01), respectively}.Patients with slow coronary flow have increased mean platelet volume which was associated with insulin resistance in non-diabetic slow coronary flow patients. TIMI frame counts correlated with mean platelet volume and increased insulin resistance. Thus, insulin resistance and platelet activity may have a role in the pathogenesis of slow coronary flow. Also, they may have a possible benefit as follow-up markers in non-diabetic patients with slow coronary flow.