Objective: Social phobia can be treated by brief Internet-based cognitive behaviour therapy (CBT). Most people with social phobia, however, meet criteria for another mental disorder; this comorbidity is associated with significant disability, and cases of comorbidity may be more difficult to treat. The present study examined the impact of the Shyness programme, an Internet-based treatment programme for social phobia, on comorbid symptoms of depression and generalized anxiety disorder. Method: Data from three randomized controlled trials using the Shyness programme to treat social phobia were reanalysed. The 211 subjects, all of whom met DSM-IV criteria for social phobia, were divided into four groups: (i) social phobia only; (ii) social phobia with elevated symptoms of depression; (iii) social phobia with elevated symptoms of generalized anxiety; and (iv) social phobia with elevated symptoms of both generalized anxiety and depression. The improvement in social phobia, depression and anxiety following Internet-based treatment for social phobia was measured. Results: Improvement in social phobia was seen in all groups, whether comorbid or not. Significant improvements in comorbid symptoms of depression and generalized anxiety occurred even though the treatment was focused on the social phobia. Conclusions: Brief Internet-based CBT can reduce both the target disorder as well as comorbid symptoms. These findings are consistent with evidence that unified or transdiagnostic programmes may reduce the severity of comorbid disorders and symptoms, indicating an important direction for future research.