To investigate the prevalence of mental health problems among the elderly attending Primary Care centres in the Province of Huesca (Spain). To detect factors associated with the psychopathology of the elderly, and to demonstrate that specific training for GPs (General Practitioners) in the use of psychogeriatric screening instruments significantly increases detection. Sample selection was made by means of systematic random sampling, stratified by the participants' health centres. Two hundred and ninety-three patients over 65 were assessed with the Spanish version of the Mini-Mental State Examination, Clock Drawing Test, Verbal Fluency, Informant Questionnaire (detection of cognitive impairment), Yesavage Geriatric Depression Scale, Goldberg Anxiety and Depression Scale, and Geriatric Mental State Schedule (items for delusions, hallucinations, obsessive ideas and hypochondriacal ideas). Almost half the patients (46.1%) presented some type of psychiatric symptom at the time of assessment. The most prevalent disorders were: cognitive impairment (16.4%), anxiety (15.7%), depression (14.3%), psychosis (6.1%), obsessive symptoms (4.4%) and hypochondriacal ideas (7.2%). Women presented more psychopathological disorders than men (58.7% vs 34.0%). Age, severity of physical illness, poor social support and previous psychiatric comorbidity were associated with psychiatric disorders in the elderly. Only 29% of the elderly population studied had a previous psychiatric diagnosis, so the detection of mental problems in old age is low in Primary Care. When Family Doctors are trained in psychogeriatric screening instruments, detection and sensitivity in the recognition of these important health problems increases. Greater training in psychogeriatric assessment is required in Primary Care.