AbstractBackgroundThe coronavirus disease 2019 (COVID‐19) pandemic interfered in the daily lives of people and is assumed to adversely affect mental health. However, the effects on mood (in)stability of bipolar disorder (BD) patients and the comparison to pre‐COVID‐19 symptom severity levels are unknown.MethodBetween April and September, 2020, symptoms and well‐being were assessed in the Bipolar Netherlands Cohort (BINCO) study of recently diagnosed patients with BD I and II. The questionnaire contained questions regarding manic and depressive symptoms (YMRS and ASRM, QIDS), worry (PSWQ), stress (PSS), loneliness, sleep, fear for COVID‐19, positive coping, and substance use. As manic, depressive and stress symptoms levels were assessed pre‐COVID‐19, their trajectories during the lockdown restrictions were estimated using mixed models.ResultsOf the 70 invited BD patients, 36 (51%) responded at least once (mean age of 36.7 years, 54% female, and 31% BD type 1) to the COVID‐19 assessments. There was a significant increase (X² = 17.06; p = .004) in (hypo)manic symptoms from baseline during the first COVID‐19 wave, with a decrease thereafter. Fear of COVID‐19 (X² = 18.01; p = .003) and positive coping (X² = 12.44; p = .03) were the highest at the start of the pandemic and decreased thereafter. Other scales including depression and stress symptoms did not vary significantly over time.ConclusionWe found a meaningful increase in manic symptomatology from pre‐COVID‐19 into the initial phases of the pandemic in BD patients. These symptoms decreased along with fear of COVID‐19 and positive coping during the following months when lockdown measures were eased.