The neuropsychological performance and Magnetic Resonance Imaging (MRI) brain appearance of a consecutive series of 46 in-patients with anorexia nervosa (AN) was compared with hat of 41 normal-weight controls. The groups were matched for sex, age, estimated pre-morbid intelligence and education. AN patients who had gained at least 10% of their body weight were retested and rescanned. Controls were retested after a similar interval. The AN group performed significantly worse than the controls on tasks measuring attention, visuospatial ability and memory. On tasks assessing flexibility and learning, no group differences were evident although an examination of deficits in individuals revealed that more anorexics were impaired on both. Following treatment, the AN group improved relative to the control group only on those tasks assessing attention. Comparison of MRI measures showed a significant proportion of anorexics had enlarged lateral ventricles and dilated sulci on both cortical and cerebellar surfaces, but no dilatation was evident for the third and fourth ventricular measures. Improvements were found after treatment on some of the radiological measures but many differences remained. Relationships between morphological brain changes and cognitive impairments were weak. Lower weight, but not duration of illness, was associated with poorer performance on tasks assessing flexibility/inhibition and memory, and with greater MRI ventricular size.