Objective: An important question in pediatric bipolar research is whether marked nonepisodic irritability is a manifestation of bipolar disorder in youth. This study tests the hypothesis that youth with severe mood dysregulation (SMD), a category created for the purpose of studying children presenting with severe nonepisodic irritability, will be significantly less likely to develop (hypo-)manic or mixed episodes over time than will youth with bipolar disorder (BD). Method: Patients with SMD (N = 84) and narrowly defined BD (N = 93) at baseline were followed up in 6-monthly intervals using the relevant K-SADS modules to ascertain (hypo-)manic or mixed episodes. Results: Only one of 84 SMD subjects (1/84 [1.2%]; 95% confidence interval CI = 0.0003 to 0.064) experienced a (hypo-)manic or mixed episode during the study (median follow-up = 28.7 months). The frequency of such episodes was more than 50 times higher in those with narrowly defined BD (58/93 [62.4%]; 95% CI 0.52 to 0.72). Conclusions: These data suggest that, over an approximately 2-year follow-up period, youth with SMD are unlikely to develop (hypo-)manic or mixed episodes. (Contains 2 figures and 2 tables.)