OBJECTIVE We carried out a prospective longitudinal study of 41 patients diagnosed as probably having a mild form of Alzheimer's disease according to the data of an investigation of prevalence done in 1991 in the municipality of Habana Vieja. MATERIAL AND METHODS The research covered two phases, with an interval of one year between them. During both phases the patients were interviewed by the same neurology resident in a door-to-door survey using the Mini-Mental State, Hughes scale (CDR) and Blessed scale, to evaluate higher mental functions. We determined the progress of the disease over the course of two years (1991-1992 and 1992-1993) and the frequency and degree of deterioration of cognitive functions during a period of one year. RESULTS AND CONCLUSIONS There was evidence of progressive worsening of the illness in 46.4% of the patients. In 34.2% this was to a moderate form and in 12.2% to a severe form. There was no progression in 46.3%. In this group 17.0% continued with a diagnosis of doubtful dementia and 29.3% as having slight dementia. The other 7.3% of the total number of patients (n = 41) were reclassified as normal. Cognitive functions almost always showed a tendency to deteriorate over time, but in a small percentage of patients they did not deteriorate and some even improved. The cognitive functions which deteriorated most were those of orientation, language and copying, with an average deterioration of 28% and 24% respectively with regard to their initial values.