BACKGROUND: This study examined the prevalence of irritability in patients with bipolar I disorder during an episode of Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV) major depression who do not meet criteria for a mixed episode. METHOD: A chart review of 111 patients with bipolar I disorder treated at the Massachusetts General Hospital Bipolar Clinic between 1998 and 2000 identified 34 patients who met criteria for a DSM-IV major depressive episode in the absence of (1) mood elevation and/or (2) irritability associated with any additional above threshold DSM-IV symptoms of mania. Data gathered from the charts utilized prospective ratings made routinely at each clinic visit using the Clinical Monitoring Form (CMF), a structured assessment instrument which includes modified versions of the mood modules of the Structured Clinical Interview for DSM-IV. Data from these 34 patients were reviewed to determine the presence of irritability. RESULTS: The frequency of abnormal irritability in these 34 patients followed a bimodal distribution: 26% of the patients showed abnormal irritability > or =75% of the time, compared with 68% of the patients with abnormal irritability < or =30% of the time. Of the high-irritability patients, psychomotor agitation was rated as definitely present to a significant degree in 44%. Talkativeness and distractibility were rated present but subthreshold in one patient each. All other symptoms of DSM-IV mania were absent. CONCLUSION: Approximately 25% of patients with bipolar I disorder who meet criteria for a DSM-IV major depressive episode also experienced substantial irritability in the absence of associated symptoms of mania. Our results suggest that abnormal irritability is not limited to mania or mixed states.