A case is presented of a breast-feeding mother receiving meropenem treatment for a postpartum urinary tract infection caused by extended-spectrum beta-lactamase producing Escherichia coli. Five milk samples were collected in a 48-hour period during meropenem therapy. The average and maximum meropenem concentrations in milk were 0.48 and 0.64 µg/mL, respectively. Based on the maximum concentration, the calculated infant daily exposure from breast milk was 97 µg/kg/d, and the infant weight-adjusted percentage of maternal dosage was 0.18%. There were no dermatologic or gastrointestinal side effects noted in the breastfed infant. Meropenem appears to be acceptable to use during breast-feeding.