Three chronic pedophiliac sex offenders were treated individually with medroxyprogesterone acetate (MPA) over a minimum of 3 months. Genital and subjective response to erotic stimulation, nocturnal penile tumescence, self-reporting of sexual urges, and testosterone levels were recorded repeatedly throughout the study. A single-subject reversal design was used and medication was administered through a double-blind procedure. The results showed that self-report of arousal outside of a laboratory setting was unreliable as a measure of the drug effect. In a laboratory setting, however, there appeared to be a significant reduction in the report of arousal to erotic stimuli while genital arousal decreased only slightly. Reversal of these responses occurred in only one subject during a final placebo phase. Nocturnal penile tumescence was significantly decreased during MPA administration and appeared to be related to decreases in total testosterone.