Although each paper reviewed for this article examined different populations, used differing measures to detect psychiatric problems and differing criteria to diagnose psychiatric illness, the results consistently demonstrate the high prevalence of psychiatric illness in the community and in primary care. Twenty-five per cent of all attenders, 52% of high utilizers and 46% of illness initiation visits in primary care have a diagnosable psychiatric illness. Depression and anxiety appear to be the most frequently encountered psychiatric problems in primary care patients. Also consistent in the three primary care based reports is the finding that primary care physicians under-diagnose psychiatric illness in their patients. Reasons for this appear to include: (i) the relatively short duration of primary care visits (though against this is the frequency of visits-particularly among high utilizers), (ii) the presentation of somatic symptomatology by patients, (iii) the shortcomings of current diagnostic categories when applied to primary care patients, (iv) the frequency of relatively mild and subthreshold conditions in primary care patients, and (v) the lack of evidence demonstrating beneficial treatment outcomes for primary care psychiatric disorders and the related difficulty of extrapolating treatment results from hospital and referral based trails to primary care patients. Despite these difficulties, family physicians need to be watchful to detect psychiatric illnesses in those patients most likely to benefit from treatment. The finding of Broadhead et al. indicating that persons with symptoms of depressed mood are at increased risk for development of major depression over the subsequent 12 months is of interest to family physicians. More frequent follow-up of this group may be indicated.(ABSTRACT TRUNCATED AT 250 WORDS)