Overlap between social phobia (SP) and panic disorder (PD) has been observed in epidemiological, family, and challenge studies. One possible explanation is that some cases of SP develop as a consequence of a panic attack in a social situation. By definition, these cases of SP have sudden onset. It is hypothesized that patients with SP with sudden onset are more similar to patients with comorbid SP and PD than to patients with SP without sudden onset regarding age of onset, extraversion, and prevalence of anxiety symptoms. One hundred and eighty-two patients with a lifetime diagnosis of PD and/or SP were recruited as part of an etiological study. Patients with SP with sudden onset did, as hypothesized, differ from patients with SP without sudden onset with regard to age of onset and extraversion, but not with regard to symptoms. They did not differ markedly from patients with comorbid SP and PD. The concept of post-panic SP is discussed.