Meta-analytic reviews of placebo-controlled studies for obsessive-compulsive disorder have found that clomipramine is more effective than drugs with more selective actions on serotonin reuptake, whereas in most direct comparisons, clomipramine's superiority has been less obvious. The authors used metaregression to identify sources of he-terogeneity in placebo-controlled trials of clomipramine, fluvoxamine, sertraline, and paroxetine. They evaluated such patient characteristics as age, gender, age of obsessive-compulsive disorder (OCD) onset, and baseline severity of OCD and depression, and such study characteristics as exclusion or inclusion criteria, length of single-blind prerandomization period, length of trial, number of subjects, and publication year. We found considerable heterogeneity across studies that was associated, in part, with publication year, length of single-blind prerandomization period, length of trial, and severity of patients' OCD. The apparent superiority of clomipramine persisted after controlling for these factors. The authors also confirmed previous reports that placebo response is higher in more recent studies. Meta-analyses can help characterize responders and nonresponders. The authors urge investigators to provide summaries of patient characteristics, especially baseline severity, age at onset, and duration of OCD, by patients' response.