Objective: The aim of this study was to investigate if testosterone undecanoate in patients with hypogonadism attenuates anemia and the risk for cardiovascular disease. Design and Method: A registry study consisted of 58 participants with subnormal total testosterone level (<2.35 ng/ml) and at least mild symptoms of testosterone deficiency. All the patients were injected with 1,000 mg of testosterone undecanoate on initial visit, followed by injection at 6, 18, 30, 42 and 54 weeks. Serum hormones, Hb, Hct, anemia and inflammation relating factors, lipid profiles, whole blood viscosity and anthropometry were measured. Results: Total testosterone (from 1.87 ± 1.09 to 5.52 ± 1.92 ng/ml, p <0.001) and free testosterone (p <0.001) were restored by testosterone undecanoate therapy. Hb and Hct significantly increased after testosterone undecanoate therapy by an average of 2.46 g/dl (p <0.001) and 3.03% (p <0.001), respectively. Prevalence of anemia (from 29.6 to 10.0%) and total cholesterol (p = 0.002), hs-CRP (p <0.001), ESR (p <0.001) significantly decreased (p <0.001) and patients with anemia showed a significant increase in erythropoietin (p = 0.047) after testosterone undecanoate therapy. Higher whole blood viscosity and increased Hct were observed until 54 weeks compared with baseline, however whole blood viscosity and Hct stabilized after 18 weeks. Conclusions: 54-week testosterone undecanoate decreased the prevalence of anemia and components of metabolic syndrome. A longer duration testosterone undecanoate therapy of more than 18 weeks may be effective and safe in reducing the blood viscosity and improving anemia in the patients with hypogonadism