This study was to investigate the prevalence of dementia in an aging population. A two-phase model was used to obtain information on the socio-demographic, medical and cognitive status of subjects over 65 years of age (n = 516), resident on December 31, 1990, within the general population (n = 3,457) of La Selva del Camp. A diagnostic protocol, following the criteria of DSM-III, was designed for application to all subjects. We diagnosed 64 subjects with dementia, which represented a prevalence of 14.9% of which 3.2% was classified as severe, 4.5% as moderate and 7.3% as slight. The prevalence by age and sex showed a large increase with age and a higher prevalence in females, although the latter was not statistically significant.