Selective serotonin reuptake inhibitors (SSRIs) are the gold standard for the pharmacological treatment of generalized social phobia (GSP). However, little is known about the predictors of response to treatment. Two hundred and four outpatients with GSP were randomized to sertraline (Zoloft) or placebo, for a 20-week double-blind study, with a flexible dose range of sertraline 50 to 200 mg/d. Response was defined as the percentage of patients with a Clinical Global Impression-Improvement scale (CGI-I) of 1 (very much improved) or 2 (much improved). Outcome analyses were conducted using regression models including treatment group as a categorical predictor and study visit as a repeated measure. Dependent measures included Marks Fear Questionnaire (MFQ), Brief Social Phobia Scale (BSPS), CGI-I, and Sheehan Disability Scale (SDS). We investigated several possible predictors of response to treatment including DSM-IV comorbidity, age, sex, age of onset of GSP, and duration of illness. Patients with later-onset (especially adult-onset) GSP tend to have a better response to treatment than those with earlier-onset GSP. This result generally appears in our analyses as a 2-way interaction, where the association with response is greatest for patients with adult-onset GSP (in contrast to those with child or adolescent onset). This finding is most robust for symptom measures, but is still apparent for the Sheehan measure of disability at work. This advantage for later-onset GSP can be accounted for neither by severity of illness nor by duration of illness. Superior treatment outcome for later-onset GSP may be mediated by the degree of social and family disability.