Brazil has been experiencing a steady increase in the elderly population during the past few years, and as a result old age health-related problems are increasing continuously in number. Psychiatric symptoms are among the most prevalent health problems of the elderly and are an important source of distress for patients and carers, being also associated with significant growth in the costs and demand for the provision of health care services. This study aimed to investigate the prevalence of mental health problems among the elderly attending a regional primary care unit in the city of São Paulo, Brazil. A total of 351 patients older than 60 were assessed during a 6-month period with the SRQ-20 (a scale for the detection of minor psychiatric problems), four questions on psychotic symptoms. AMTS (a scale for the assessment of cognitive functioning) and CAGE (a scale for the assessment of alcoholism); demographic variables were also recorded. Subjects' mean age was 71.22 (CI = 70.51-71.92) and 83.5% were female. Thirty-two per cent of subjects were considered 'cases' as they scored more than 7 on the SRQ-20 (26.8% of total), or more than 1 on the CAGE (1.4% of total), or at least 1 on the questions assessing psychotic symptoms (12.2% of total). There was a significant excess of women among those found to suffer from psychiatric problems (90.1% vs 80.4%). Cases were also more likely to be illiterate (23.4% vs 12.1%) and to have a lower income (2.21 vs 4.01 minimum wages). Depressive symptoms and tension were highly prevalent (72.9% of subjects). Somatic complaints and signs of inefficient functioning were also common (50.4% and 45.9% respectively). Only five patients answered affirmatively two or more CAGE questions and 12.2% scored at least 1 on the questions assessing psychotic symptoms. Cognitive deficit, as determined by the AMTS, was observed in 13.4% of the sample and was associated with ageing, being illiterate and having higher total SRQ-20 scores. We suggest, that the organization of health care services should take into account the needs of this population, and should also emphasize professional training for the correct assessment and treatment of the most frequent mental health problems in old age.