Low testosterone level is common in obese men and even more frequent in the presence of type 2 diabetes mellitus. Low testosterone level is probably not caused by increased aromatization of testosterone to estradiol in fat tissue. Increase of inflammation mediators, insulin and leptin resistance and low SHBG level may play more important role. Low testosterone level may manifest with low libido, erectile dysfunction, fatigue and depressive mood; it has a role in development of anaemia, osteoporosis, worsening of insulin resistance and it is probably marker of increased all-case and cardiovascular mortality. Routine investigation of testosterone level is indicated in every man with type 2 diabetes mellitus. As in non-diabetic men of middle and higher age substitution is indicated only in presence of clinical symptoms of hypogonadism. The substitution improves mainly libido, erectile dysfunction is influenced less. However, the phosphodiasterase-5 inhibitors efficiency is markedly improved. Metabolic changes during testosterone substitution are small.