Lithium is a drug of choice for the management of bipolar disorder, a disease frequently affecting women in their childbearing years. Unfortunately, this drug has typically been contraindicated in nursing women. Data in humans are limited with respect to the use of this drug in lactating women, and early reports suggest high excretion into milk. The purpose of this report was to verify the excretion of lithium into human milk and to assess infant safety after breast-feeding. The authors found wide interpatient variability in lithium dose offered to the infant through breast milk (from 0% to 30% of maternal weight-adjusted dose), indicating that therapeutic drug monitoring of lithium in milk and/or in infant's blood, coupled with close monitoring of adverse effects, is a rational approach. Since therapeutic drug monitoring of lithium is routine, physicians caring for these women and infants should be encouraged to individualize their recommendations.