We hypothesised that hippocampal volume would be reduced in underweight anorexia nervosa (AN) and associated with impaired hippocampus-dependent cognitive function. Hippocampal and whole brain volumes were measured in 16 women with AN and 16 matched healthy women using magnetic resonance imaging (MRI) and a manual tracing method. Participants also completed the Doors and People Test of hippocampus-dependent memory and an IQ test. After adjustment for total cerebral volume, there was significant bilateral reduction in hippocampal volume in the AN group (8.2% right; 7.5% left). There was no evidence of impaired hippocampus-dependent cognitive function and no evidence of a relationship between hippocampal volume and clinical features of AN. The reduced hippocampal volume in anorexia nervosa is not associated with changes in cognitive function. To understand the cause and consequence of hippocampal size and function, it will be important to integrate endocrine, neuropsychological and neuroimaging studies.