Early stage Alzheimer disease patients and matched elderly unaffected controls (n = 16/group) were evaluated with the P300 event-related brain potential (ERP). All subjects performed four oddball tasks that varied systematically in task difficulty and were each presented in the auditory and visual modalities. P300 amplitude was smaller and peak latency longer for the Alzheimer patients compared to elderly control subjects across tasks and modalities. P300 differences between Alzheimer patients and controls were largest for the relatively easy tasks, with little influence of stimulus modality observed. The results suggest that the P300 brain potential is sensitive to Alzheimer's disease processes during its early stages, and that easily performed stimulus discrimination tasks are the clinically most useful. Theoretical and practical implications are reviewed.