Recently, testosterone undecanoate (TU), a new parenteral testosterone (T) preparation has been introduced. Two of its distinctive features are (a) its prolonged action: after two initial loading injections 6 weeks apart, usually only one injection every 12 weeks is needed (b) over the full interval between two injections, plasma T levels are in the physiological range. New research presents convincing evidence that T has profound effects on tissues of the penis involved in the mechanism of erection and that testosterone deficiency impairs the anatomical and physiological substrate of erectile capacity, which is, at least, in part reversible upon androgen therapy. Our studies with TU demonstrated that venous leakage could be corrected with T treatment in a number of patients. We further could show that sexual functions, in a substantial number of elderly men, can be restored with treatment with T only. So, these results argue for determination of T levels in elderly men with sexual problems. If the levels are subnormal, T treatment is warranted.