This prospective case–control study aimed to test the presence of insulin resistance (IR) in men with unexplained infertility. We included two groups: the study group including 160 infertile men with unexplained oligozoospermia (sperm count <10 × 106/ml) and normal hormonal profile, and the control group of 79 men with proven fertility within the preceding year. A fasting blood test measured IR, FSH, LH, total cholesterol, low-density lipoprotein, high-density lipoprotein and triglycerides. Insulin level was significantly higher in the study group (13.67 ± 10.44) compared with the control group (5.46 ± 3.15), P < 0.0001, and IR was significantly higher in the study group, P < 0.0001. FSH was significantly (P < 0.0001) higher in the study group (4.71 ± 2.57) than the control group (3.15 ± 1.92). LH was significantly higher in the study group (4.98 ± 2.41) compared with the control group (3.15 ± 1.12), P < 0.0001. Total cholesterol was significantly higher in the study group (198.29 ± 37.52) than the control group (182.45 ± 35.92), P < 0.05. In conclusion, IR in men with unexplained infertility may be a cause of reproductive and metabolic abnormalities. The benefit of insulin-sensitizing agents for these patients should be tested.