Conducted a meta-analysis of 66 published studies that compared the psychological adjustment of adoptees and nonadoptees. The mean within-study effect size was .72, indicating that adoptees had significantly higher levels of maladjustment. This was largely due to studies that found that adoptees are significantly overrepresented in clinic populations (mean effect size = 1.38). In addition, adoptees were found to have higher levels of externalizing disorders and academic problems than nonadoptees. Effect sizes tended to be larger for adolescents than for children and adults. These results are consistent with the conclusions of previous qualitative reviews of the research on the psychological adjustment of adoptees. The relationship of effect size to both methodological and subject variables is discussed, and suggestions for future research are provided.