BACKGROUND: Bipolar disorder is associated with both white matter abnormalities and hypothalamo-pituitary-adrenal axis dysfunction. In a post-hoc analysis of diffusion tensor data, the relationship between cortisol levels and white matter structural integrity was explored in healthy controls and in euthymic patients with bipolar disorder. METHODS: Healthy control subjects and patients with bipolar disorder, prospectively verified as euthymic, underwent diffusion tensor MRI: fractional anisotropy and mean diffusivity data in fifteen regions of interest were obtained. Morning and evening salivary cortisol levels (SCLs) were measured. RESULTS: Significant negative partial correlations were found between fractional anisotropy and evening SCLs in control subjects in four periventricular regions. This pattern was absent in bipolar patients, possibly due to the presence of an excess of extracellular fluid manifested as a significant increase in mean diffusivity in those regions. LIMITATIONS: This is an exploratory, post-hoc analysis of data with relatively small sample sizes. Lithium treatment and past substance abuse in the bipolar group are potentially confounding factors in this study. CONCLUSIONS: These preliminary data show an inverse relationship between evening cortisol levels and a measure of periventricular white matter integrity in healthy controls. This relationship appears disrupted in bipolar patients, possibly due to periventricular osmoregulatory dysfunction, the effects of medication or past substance use. Future research should further investigate the influences of cortisol on oligodendrocyte function, white matter integrity and brain osmoregulation in bipolar disorder.