Probability of survival was investigated in a sample of 90 patients with Alzheimer's disease of mild to moderate severity over a period of 36 months. It showed a decline of approximately 10 per cent per year of observation. This is equivalent to an excess mortality of 21 as compared with the age-matched population. The estimated survival rate was analysed with regard to the influence of sex, and of age and severity of cognitive impairment at the initial examination. The most powerful predictor for a reduced probability of survival was the degree of cognitive impairment at study entry.