In summary, evidence is presented associating typically asymptomatic and reversible elevations of serum transaminase values with high daily dosages of naltrexone. Statistical significance was found only between placebo and the 300 mg dosage. Subjects aged 40 years and over were significantly more likely to develop this finding than younger subjects. All subjects with significant elevations of transaminase values in these studies took daily naltrexone dosages higher than recommended for opioid addiction. The daily dosage of naltrexone recommended for opioid addiction did not cause abnormalities of serum transaminase values in these studies.