The influence of psychosocial stress on the course of bipolar disorder has been increasingly recognized. The authors tested hypotheses about both stress and early adversity "sensitization" on the course of bipolar disorder over a one-year period.The participants were 58 adults (29 male and 29 female) with a diagnosis of bipolar I disorder. They were evaluated every three months for one year. Stressful life events and the presence of early adversity were assessed by structured interview.There was no significant interaction between stress and episode number in the prediction of bipolar recurrence. The interaction of early adversity severity and stressful life events significantly predicted recurrence in a manner consistent with the sensitization hypothesis. Participants with early adversity reported lower levels of stress prior to recurrence than those without early adversity. Individuals with early adversity also had a significantly younger age of bipolar onset.The sample size was small and the number of past episodes was determined retrospectively, mainly through self-report.Severe early adversity may result in a greater effect of stress on bipolar recurrence and earlier onset of bipolar disorder, suggesting the need for further studies of stress mechanisms in bipolar disorder and of treatments designed to intervene early among those at risk.