Invasive breast cancer after initiation of testosterone replacement therapy in a man--a warning to endocrinologists.

Objective: To alert fellow endocrinologists of a rare side effect of testosterone therapy, for which men with hypogonadism must receive appropriate counseling and monitoring.

Methods: We present clinical features, laboratory data, and histopathologic findings in a man with hypogonadism who received testosterone replacement therapy.

Results: A 61-year-old man was referred to an endocrinologist after presenting to his general practitioner with erectile dysfunction and low libido.