Background The main objectives of the survey were: (a) to analyse the sociological, clinical and illness-related correlates of mental illness in primary care; (b) to study, during one-year follow-up, outcome and use of health resources. Method The survey comprised a two-phase cross-sectional study. In the first phase patients were classified using the GHQ-28 or by the general practitioner (GP). In the second phase they were assessed by the SCAN system. Results The prevalence rate of mental illness (in attenders) using the GHQ was 33.2%. The corresponding rate for the GP was 14.1%, and for the SCAN 31.5%. Mental illness mainly comprised depression, anxiety and alcohol-related diagnoses. The presence of mental illness and the use of health resources during follow-up were dependent on demographic characteristics and on their original psychiatric status. Conclusions In primary care, mental illness constitutes a major health problem. Despite this fact. GPs do not recognise a substantial proportion of these health problems.