Naltrexone treatment, used to prevent relapse among former opioid addicts, is reported to have an extraordinary rate of noncompliance. Since activation of opioid receptors produces a sense of well-being, naltrexone's blockade of these receptors might produce dysphoria, which could contribute to noncompliance among addicts under treatment. To test this hypothesis, the authors administered naltrexone to four men who had been free of opioids for 9 to 44 months using a 6-week, placebo-controlled crossover design. One subject dropped out with abstinence-like symptoms, and two others reported mild but significantly greater dysphoria during naltrexone administration. The results suggest that naltrexone may induce mild dysphoria long after addicts stop using opioids.