Hippocampal subfields CA3 and CA1 are hypothesized to differentially support the generation of associative predictions and the detection of associative mismatches, respectively. Using high-resolution functional MRI, we examined hippocampal subfield activation during associative retrieval and during subsequent comparisons of memory to matching or mismatching decision probes. Activity in the dentate gyrus/CA2/3, CA1, and other medial temporal lobe subregions tracked associative retrieval success, whereas activity in CA1 and the perirhinal cortex tracked the presence of associative mismatches. These data support the hypothesis that CA1 acts as a "comparator," detecting when memory for the past and sensory input in the present diverge.