The aim of this 7-year inception cohort study was to determine the prognostic value of plateaux in cognitive decline in the course of Alzheimer's disease (AD) as well as their impact on the rate of progression of cognitive impairment. From a consecutive sample of 106 outpatients participating in a longitudinal study on AD, we selected 31 with a mild degree of mental deficit at presentation and a disease duration of at least 3 years when included into the study. All underwent extensive clinico-neuropsychological testing about every 6 months and there were no drop-outs. Mean period of follow-up lasted 6.8 (SD 2.9) years. Nineteen patients displayed a plateau, where a plateau refers to a patient's remaining on a mild level of cognitive decline for more than two years. Survival curves (Kaplan-Meier method) showed that patients with plateaux reached several end-points--very severe functional or cognitive impairment, urinary incontinence, death--significantly later than patients without (p < 0.04). Patients with plateaux showed a smaller cognitive loss (p < 0.01) in terms of the mean annual rate of progression of mental decline. In conclusion plateaux in an early stage of Alzheimer's Disease served to identify patients with a more favourable course.