Age at onset, duration, and severity of dementia were evaluated in 165 patients with a clinical diagnosis of Alzheimer's disease. Rate of progression of dementia was determined in 77 patients by repeated administration of the Blessed Dementia Scale (BDS). The distribution of age onset among patients was bimodal, with a division at about age 65. Duration of dementia at the time of initial examination was shorter, and rate of progression on follow‐up examination was more rapid in senile‐onset (age 65 or greater) than in presenile‐onset (before age 65) cases. Considerable overlap among values for the two patient groups was observed for both variables, indicating that age at onset is not a strong predictor of rate of progression of dementia in patients with Alzheimer's di