The purpose of this study was to examine patterns of comorbidity of posuraumatic stress disorder (PTSD) and social phobia (SP) and explore factors associated with the etiology and effect of this comorbidity. The sample consisted of 443 patients seeking treatment of PTSD, SP, or both. Of 240 patients with principal PTSD, 43% had comorbid SP, whereas only 7% of 57 with principal SP had comorbid PTSD. Patients with principal PTSD and comorbid SP were more likely to meet criteria for major depression and other anxiety disorders, and reported worse depression, anxiety, and physical, mental, and social functioning, than those with only PTSD or SP, regardless of other anxiety comorbidity. PTSD patients with SP reported more trauma-related guilt and higher frequency of childhood abuse than those without SP. The results are discussed in terms of implications for treatment of the comorbid presentation and directions for future research.