BACKGROUND Sexual dysfunction is one of the most common problems in men after coronary artery bypass graft (CABG). Etiology of sexual dysfunction in these patients may be psychological or organic due to hormonal changes. OBJECTIVE The purpose was to evaluate the incidence and type of sexual dysfunction and changes in serum concentration of sex hormones in male patients undergoing on-pump CABG. MATERIALS AND METHODS In this before and after study we enrolled 40 men aged less than 70 years who were candidate for on-pump CABG. Interviews were done by a physician before and 12 weeks after the operation in regard to the impact of surgery on their sexual activities. The serum levels of 6 sex hormones were also determined. The statistical tests used for data analysis included analysis of variance, McNemar's test and chi-square analysis. RESULTS The mean±SD age of the patients was 51.27±7.86 years. Incidence of sexual dysfunction was 22.5% (9 cases) before and 47.7% (19 cases) after operation. Types of sexual dysfunction were premature ejaculation (5% before, 2.5% after), impotence (7.5% before, 12.5% after) and loss of libido (10% before, 32% after). The level of sex hormones were generally decreased after operation but it was statistically significant only for estrogen (p-value=0.02). CONCLUSION Sexual dysfunction and reduction in serum level of sex hormones are common in patients before on-pump CABG and mostly get worse after surgery. Complementary studies are suggested for prevention and treatment of sexual dysfunction.