Objective Magnetic resonance imaging ( MRI ) studies in bipolar I disorder ( BD ‐I) suggest that lithium is associated with increased volumes of cortico‐limbic structures. However, more rigorous control of confounding factors is needed to obtain further support for this hypothesis. The aim of the present study was to assess differences in brain volumes among long‐term lithium‐treated BD ‐I patients, unmedicated BD ‐I patients, and healthy controls. Methods This was a cross‐sectional study with 32 euthymic BD ‐I patients (16 on lithium monotherapy for a mean of 180 months, and 16 receiving no medication for at least the 2 months prior to the study) and 20 healthy controls. Patients were euthymic (Hamilton Depression Rating Scale [ HDRS ] <6 and Young Mania Rating Scale [ YMRS ] <7) and had not taken psychotropic medications other than lithium for at least 6 months. Brain images were acquired on a 1.5 Tesla MRI (Phillips, Amsterdam, The Netherlands) and segmented to generate volumetric measures of cortical and subcortical brain areas, ventricles and global brain. Results Significant differences were found in the volumes of the left amygdala ( P =.0003), right amygdala ( P =.030), left hippocampus ( P =.022), left thalamus ( P =.022), and right thalamus ( P =.019) in long‐term lithium‐treated BD ‐I patients, compared to unmedicated patients and controls, after multivariable adjustment. No differences were observed in global brain volume or in ventricular size among the three groups. Likewise, there was no correlation between serum lithium levels and the increase in size in the described brain areas. Conclusions The structural differences found among the three groups, and specifically those between long‐term lithium‐treated and unmedicated BD ‐I patients, indicate increased limbic structure volumes in lithium‐treated patients.