Context Clinical guidelines recommend measurement of the serum prostate-specific antigen (PSA) concentration during testosterone treatment of hypogonadal men to determine whether the increase is sufficiently high to warrant urologic referral. Prior studies of the effect of testosterone treatment on PSA concentrations have been conducted in men who were mildly to moderately hypogonadal. Objective The objective of this work is to determine the PSA response to testosterone treatment of men who are severely hypogonadal. Design and Setting This retrospective cohort study was conducted at a single academic medical center. Participants Eighty-five men participated who were severely hypogonadal as a result hypothalamic-pituitary or testicular disease. Main Outcome Measure Changes in serum PSA concentrations were measured during testosterone treatment for up to 18 months. Results Testosterone treatment increased the median serum testosterone concentration from 36 ng/dL (interquartile range [IQR], 20-91 ng/dL) at baseline to 395 ng/dL (IQR, 266-542 ng/dL) at 6 to 18 months. This treatment resulted in a median increment in PSA above baseline of 0.70 ng/mL (IQR, 0.10-1.85 ng/mL) at 6 to 18 months. Apropos current Endocrine Society clinical guidelines, 31% of the men experienced a PSA increase above baseline greater than 1.4 ng/mL, and 13% reached an absolute PSA concentration of greater than 4.0 ng/mL. Four men were diagnosed with prostate cancer. Conclusions The PSA response to testosterone replacement in men who are severely hypogonadal as a result of pituitary or testicular disease is greater than that previously reported in men with mild to moderate hypogonadism. These results suggest the magnitude of the PSA response to testosterone replacement is related to the degree of hypogonadism.