Women with a history of major depressive disorder (MDD) have increased risks for postpartum depression, but less is known about postpartum mania in this population. The objectives of this study were to prospectively determine the frequency with which mania occurs in the postpartum among women who have a history of MDD and to explore temporal relationships between onset of mania/hypomania and depression. We administered the Structured Clinical Interview for DSM IV disorders (SCID) to pregnant women with a self-reported history of MDD to confirm diagnosis and exclude women with any history of mania/hypomania. Participants completed the Edinburgh Postnatal Depression Scale and Altman Self-Rating Mania Scale (ASRM) once during the pregnancy (∼26 weeks) and 1 week, 1 month, and 3 months postpartum. Among women (n = 107) with a SCID-confirmed diagnosis of MDD, 34.6 % (n = 37) experienced mania/hypomania (defined by an ASRM score of ≥6) at ≥1 time point during the postpartum, and for just over half (20/37, 54 %), onset was during the postpartum. The highest frequency of mania/hypomania (26.4 %, n = 26) was at 1 week postpartum. Women who experienced mania/hypomania at 1 week postpartum had significantly more symptoms of mania/hypomania later in the postpartum. A substantive proportion of women with a history of MDD may experience first onset of mania/hypomania symptoms in the early postpartum, others may experience first onset during pregnancy. Taken with other recent data, these findings suggest a possible rationale for screening women with a history of MDD for mania/hypomania during the early postpartum period, but issues with screening instruments are discussed.