Background: The advent of Internet-based self-help systems for common mental disorders has generated a need for quick ways to triage would-be users to systems appropriate for their disorders. This need can be met by using brief online screening questionnaires, which can also be quickly used to screen patients prior to consultation with a GP. Objective: To test and enhance the validity of the Web Screening Questionnaire (WSQ) to screen for: depressive disorder, alcohol abuse/dependence, GAD, PTSD, social phobia, panic disorder, agoraphobia, specific phobia, and OCD. Methods: A total of 502 subjects (aged 18 - 80) answered the WSQ and 9 other questionnaires on the Internet. Of these 502, 157 were assessed for DSM-IV-disorders by phone in a WHO Composite International Diagnostic Interview with a CIDI-trained interviewer. Results: Positive WSQ "diagnosis" had significantly (P<.001) higher means on the corresponding validating questionnaire than negative WSQ "diagnosis". WSQ sensitivity was 0.72 - 1.00 and specificity was 0.44 - 0.77 after replacing three items (GAD, OCD, and panic) and adding one question for specific phobia. The Areas Under the Curve (AUCs) of the WSQ's items with scaled responses were comparable to AUCs of longer questionnaires. Conclusions: The WSQ screens appropriately for common mental disorders. While the WSQ screens out negatives well, it also yields a high number of false positives. © Tara Donker, Annemieke van Straten, Isaac Marks, Pim Cuijpers.