The aim of this study was to replicate and extend previous work demonstrating selective impairment of semantic, but not episodic, memory in late versus early onset Alzheimer's disease (AD). Measures of episodic and semantic memory were administered to 12 pairs of patients; early (less than or equal to 62) and late (greater than or equal to 68) onset pairs were matched on dementia severity and education. As hypothesized, the groups did not differ on the three episodic memory measures but did on two of three semantic memory measures. In conjunction with prior research, these findings indicate that late onset AD is characterized by more profound impairment on measures of semantic processing.