OBJECTIVE: The symptom-complex irritability, widely used in descriptions of bipolar patients' manic and mixed states, also represents a common feature in depressive phases. Irritability negatively affects the clinical course of depression, leading to a higher risk of treatment non-adherence, violence, and suicide attempts. Nevertheless, proportional attention from the scientific literature seems to be scarce. We conducted the first randomised controlled trial with the aim of evaluating BLT as a possible therapeutic strategy for irritability in bipolar depression. METHODS: 180 inpatients were randomly assigned to: Group A exposed to bright light therapy (BLT) daily, or Group B treated with pharmacotherapy only. A qualitative assessment of irritability was performed after a 4-week program. RESULTS: Group A showed about one-third fewer cases of irritability compared to Group B, this reduction was not related to the overall remission of depressive symptoms. CONCLUSIONS: The present study supports the usefulness of BLT in irritability in bipolar depression. Irritability is an underestimated feature of bipolar depression.Irritability is related to higher suicide risk and lower quality of life.Bright light therapy is an effective strategy to reduce irritability in bipolar depression.