A randomized controlled trial was conducted to examine the clinical effectiveness of providing general practitioners (GPs) with the results of a self-administered computerized assessment of common mental disorders. Attenders at a general practice in a deprived inner city area of South London were identified using case finding questionnaires. Six hundred and eighty-one subjects were randomly allocated to three groups which differed in the information provided to the GP: 1) no additional information was given to the GP; 2) the results of the 12 item General Health Questionnaire (GHQ) were given to the GP (the GHQ is a paper and pencil questionnaire that assesses common mental disorders); 3) the results of a self-administered computerized assessment (PROQSY) of common mental disorders were provided for the GP. Clinical outcome was assessed using the 12-item GHQ. Consultations with the GP, prescriptions and referrals within and outside the practice were also recorded. The group in whom the GP received the results of the computerized assessment showed a modest clinical improvement, relative to the other two groups after 6 weeks. There was no difference in clinical outcome between the groups at 6 months. There appeared to be no increase in consultations or prescriptions in the computerized assessment group. Self-administered computerized assessments for psychiatric disorder have potential as a means of improving the clinical outcome of patients in primary care. It is likely that the effectiveness of the approach would be greatly increased by linking the results of computerized assessments to clinical practice guidelines, tailored to the individual patient by means of computerized technology.