The aim of this study was to provide data to aid decision making regarding lithium use during lactation. Three women treated with lithium for bipolar disorder during pregnancy and lactation and their four infants provided lithium levels at 1 month postpartum. Infant levels ranged from 10% to 17% of maternal levels. Two infants experienced early feeding problems which were overcome with breastfeeding education and support. Women taking lithium can be supported to breastfeed, and their infants should be followed closely until breastfeeding is well established.