We validated the Hypomania Interview Guide-Seasonal Affective Disorder version (HIGH-SAD) in patients with rapid cycling bipolar disorder (RCBD). Fourteen outpatients were rated on six separate occasions (total = 84 visits). On each visit the patients were rated with the HIGH-SAD and the Young Mania Rating Scale (YMRS) in a counterbalanced order. Clinical assessment was completed at the end of the visit by the treating psychiatrist. Patients were assessed as hypomanic/manic on 22 of the visits. Pearson correlation coefficient between the YMRS total scores and the HIGH-SAD total scores for those 22 visits in which patients were hypomanic/manic was r = 0.629 (P < 0.05) and for all visits was r = 0.769 (P < 0.0001). Analysis with only one rating per patient yielded a Pearson correlation coefficient of r = 0.792 (P < 0.0004). We found that the HIGH-SAD was a valid scale for the measurement of hypomania in patients with RCBD. However, the scale does not differentiate hypomania from mania in this group of patients.