Hypogonadism can be of hypothalamic-pituitary origin or of testicular origin, or a combination of both, which is increasingly common in the aging male population. In the postpubertal male, testosterone replacement therapy can be used to treat the signs and symptoms of low testosterone, which include loss of libido, erectile dysfunction, diminished intellectual capacity, depression, lethargy, osteoporosis, loss of muscle mass and strength, and some regression of secondary sexual characteristics. Before initiation of testosterone replacement therapy, an examination of the prostate and assessment of prostate symptoms should be performed, and both the hematocrit and lipid profile should be measured. Absolute contraindications to testosterone replacement therapy are prostate or breast cancer, a hematocrit of 55% or greater, or sensitivity to the testosterone formulation.