The prognosis for patients with Alzheimer's disease is important information for physicians to be able to provide patients and their relatives as an aid to making appropriate arrangements before the severe stage is reached. OBJECTIVES: To provide prognosis aids for patients with mild or moderate Alzheimer's disease based on the standardised Mini-Mental State Examination (SMMSE). METHODS: Data from a Canadian cohort study of 206 patients with an initial SMMSE between 10 and 24 were used to find determinants of the three year probability of reaching a highly dependent stage, defined as SMMSE < 10. The regression equations were also used to derive a reference failure-time curve. The predicted progression was compared with that observed in a US study (N=597). RESULTS: Proportional hazards analyses showed that at the mild stage (SMMSE 19 to 24) the presence of hallucinations was associated with a more rapid decline, whereas at the moderate stage (SMMSE 10 to 18) the important predictors of decline were a lower baseline SMMSE score and longer time since onset. Absence of hallucinations in patients with an SMMSE above 18, implied a 79% probability of remaining independent after three years; presence of hallucinations reduced this to 52%, while a prior rate of decline of 2 points/year did so even further to 43%. Less than half of patients whose SMMSE was already below 19 and who had symptoms for five years or longer remained independent after three years. An initial score below 14 resulted in a probability below 30%. The predictions based on the Canadian study showed reasonable agreement with the progression observed in the US study. CONCLUSIONS: These equations permit estimation of the expected progression of Alzheimer's disease, and will aid clinicians when advising patients and their caregivers about prognosis and treatment.