Testosterone and its active metabolite dihydrotestosterone exert their influence on target cells through a specific intracellular protein receptor. Structural abnormalities of this receptor lead to a diminished androgen action within the cell and result in the syndrome of androgen insensitivity. Androgen insensitivity is classified on the basis of whether the insensitivity is complete or partial and whether the androgen receptor is normally present (AR(+)), absent (AR(-)) or diminished (AR(+/-)). All patients with androgen insensitivity have normal or high plasma levels of testosterone and elevated serum LH. Patients with complete androgen insensitivity are phenotypically female. The clinical presentation of partial androgen insensitivity is variable, ranging from a minimal amount of virilization to a completely masculine appearance. All patients described with a syndrome of androgen insensitivity are infertile. The influence of androgen receptor function in the pathogenesis of benign prostatic hypertrophy is being investigated. Androgen receptor content is also being studied as a possible marker of responsiveness to hormonal therapy in prostatic carcinoma.