Objective: Callosal structural and functional alterations have been demonstrated in a range of neuropsychiatric illnesses, including bipolar disorder, but no study has examined regional callosal thickness in this phenotype. The aim of the present study was therefore to examine callosal size and shape in a well-defined group of bipolar affective disorder patients and controls. Methods: The participants included 24 patients with DSM-IV bipolar I disorder and 24 matched healthy controls. The corpus callosum was extracted from mid-callosal images from T1-weighted magnetic resonance imaging scans on all participants, and callosal area, length, bending angle and regional callosal thickness measures were computed from these images. Results: The callosum was thinner in the bipolar group overall, with a disproportionately reduced thickness in the splenium. Psychotic and non-psychotic patients did not differ, although patients without a family history of mood disorders had a thinner callosum. Conclusion: Callosal reductions are present in established bipolar disorder, and affect posterior regions more than anterior regions. This may reflect a primary disturbance to myelination, or a secondary effect of grey matter changes.