This symposium report summarizes first extensive clinical findings with injectable testosterone undecanoate (Nebido) in hypogonadal patients showing clinical symptoms of androgen deficiency with or without erectile dysfunction (ED). This new testosterone formulation (1000 mg testosterone undecanoate in 4 ml castor oil) possesses nearly ideal long-term kinetics, i.e. sustained close mimicking of eugonadal testosterone serum levels without supra- or sub-physiological serum concentrations. The generally accepted administration scheme recommends the second injection 6 weeks after the first one followed by further injections every 12 weeks. Applying this regimen, administration intervals are drastically reduced in comparison to conventional i.m. testosterone preparations (e.g. about 16 injections of testosterone enanthate vs. 4-5 injections of testosterone undecanoate per year). Depending on the testosterone serum levels, individualized therapy is possible by shortening (every 10 weeks) or prolonging (every 14 weeks) the injection intervals. In hypogonadal patients with ED 58% respond to testosterone undecanoate alone. Best results are seen in diabetic hypogonadal patients. The regimen of injectable testosterone undecanoate administration ideally fits recommendations regarding pharmacokinetics, efficacy and safety monitoring.