Previous studies have suggested that the cerebellar vermis may be smaller in individuals with bipolar disorder, but these findings are inconsistent and have not considered the potential impact of medication. To address these knowledge gaps, the cerebellar vermis was measured using structural magnetic resonance imaging in both an adolescent and an adult sample. Analysis of variance was performed on the cross-sectional area of the three vermal lobes and on the total area. There were no significant differences between adolescent or adult bipolar subjects compared to healthy controls in any region, regardless of lithium treatment. In addition, a medication-naïve subset of the adult population underwent a two-year course of lithium treatment and was then reassessed. No changes in vermis area were found within subjects across the treatment. These results, combined with literature meta-analysis, indicate no clear effect of either bipolar disorder, or lithium treatment, on the size of the cerebellar vermis.