Although many advances have been made in the treatment of Social Anxiety Disorder (SAD), less is known about its onset and factors related to its course and severity. The current study aimed to investigate developmental factors (e.g., onset of illness, behavioral inhibition, socially traumatic experiences) that research has suggested are related to the course and severity of SAD in a sample of adults diagnosed with generalized SAD. Results showed behavioral inhibition to be the only consistent predictor of current severity. Results for age of onset were consistent with previous studies suggesting an early childhood and later adolescent pattern. In addition, an earlier age of onset negatively impacted improvement in cognitive behavior therapy for SAD, but no other developmental factors were related to treatment outcome. Future research using longitudinal designs and multiple informants is needed to confirm findings from retrospective reports.