Background: Multiple epidemiological studies have shown that low testosterone levels are associated with and predict the future development of type 2 diabetes mellitus and the metabolic syndrome. The aim of our study was to show the influence of testosterone replacement therapy on obesity, HbA1c level, hypertension and dyslipidemia in patients with diabetes mellitus and androgen deficiency. Methods: One hundred and twenty-five male patients with diabetes mellitus were screened; 85 subjects aged 41 to 65 years, with BMI from 27.0 to 48.0 kg/m(2), were randomized in a placebo-controlled study. They also underwent a routine physical examination and selected by free testosterone examination. We divided patients into two groups: 1) treatment group, where we used diet, physical activity, patient's antidiabetic therapy and testosterone replacement therapy; 2) placebo group, where we used diet, physical activity, patient's antidiabetic therapy and placebo. Results: After 6 months of treatment we repeated the diagnostic assessments: lipid profile was improved in both groups but in first group it was clinically significant. Free testosterone level increased in all groups, but in group I was clinically significant. HbA1c decreased in both groups, but in group I we obtained the best result. Leptin level after treatment was approximately the same in both groups. Also, blood pressure was reduced in both groups but results were similar. Conclusions: Our study demonstrated that it is possible to break this metabolic vicious circle by raising testosterone levels in diabetic men with androgen deficiency. Re-instituting physiological levels of testosterone, as the study has shown, has an important role in reducing the prevalence of diabetic complications.