Clinical data from brain-damaged patients implicates the human hippocampal formation in memory function. We tested the hypothesis that long-term memory function is associated with activation of the hippocampal formation in humans by measuring regional cerebral blood flow changes whilst subjects performed memory tasks. Bilateral hippocampal regional cerebral blood flow was significantly correlated with a measure of the engagement of long-term auditory-verbal memory. No such association was observed for the degree of engagement of auditory-verbal subspan memory. These data provide, for the first time, direct in vivo evidence for the involvement of the hippocampal formation in long-term memory in the intact human brain.