INTRODUCTION: The aims of this study are to examine the evolution of clinical dementia diagnosis over 3 decades and to investigate secular trends of dementia. METHODS: Four cohorts covering a period from 1988 to 2013 were used: the Personnes Agées Quid and Three-City-Bordeaux studies, and the Cognitive Function and Aging Study (CFAS) I and II. Mini-Mental State Examination scores at clinical diagnosis were evaluated over a 24-year follow-up period in French studies. An algorithmic approach was applied to CFAS I and II to provide dementia prevalence and incidence estimates. RESULTS: A significant increase of the Mini-Mental State Examination score at diagnosis was observed until 2000 and a significant decrease after. We reported a prevalence of 8.8% for CFAS I (1990-1993) compared with a prevalence of 6.5% in CFAS II (2008-2011). The 2-year incidence rate was estimated at 31.2/1000 (95% confidence interval = 28.0-34.8) for CFAS I and 15.0/1000 (95% confidence interval = 13.5-16.7) for CFAS II. DISCUSSION: Applying a stable algorithm to different cohorts across time can provide a robust method for time trends estimation.