The postpartum period is considered a time of heightened vulnerability to bipolar disorder. The primary goal of this study was to examine the frequency and the polarity of postpartum episodes in a clinical sample of women with bipolar disorder who were medication-free during their pregnancies. In addition, we sought to examine whether there are differences in terms of clinical features of bipolar disorder between women with and without postpartum episodes. Lastly, we analyzed the potential relationship between polarity of the postpartum episodes and clinical features of bipolar disorder. The presence/absence of postpartum episodes and their characteristics were obtained from medical records of 276 women with bipolar disorder who were medication-free during their pregnancies. Two hundred seven women (75.0 %) had a history of one or more postpartum mood episodes: depressive (79.7 %), (hypo)manic (16.4 %), or mixed episodes (3.9 %). Psychotic symptoms during postpartum episodes were associated with depression in 37 (22.4 %) patients, with mania in 19 (67.8 %) patients, and with mixed episodes in 7 (87.5 %) patients. Postpartum manic and mixed episodes were significantly associated with type I disorder and with psychotic features. Our findings indicate high risk of clinically ascertained mood episodes during postpartum period in bipolar women who are not treated during pregnancy.