Objective: To describe a case of infant meningoencephalitis that was probably caused by yellow fever vaccine virus transmitted via breastmilk.Description: A 38-day old patient was admitted to hospital on May 23, 2009, with fever. On May 25, 2009, convulsive crises began. Cerebrospinal fluid (CSF) test results were suggestive of meningoencephalitis. The mother had been given a dose of yellow fever vaccine and the baby was on exclusive breastfeeding. The baby was discharged after the convulsive crises were controlled. Tests identified IgM antibodies specific for yellow fever in both serum and CSF. Comments:In 2009, the first case was confirmed of meningoencephalitis caused by the yellow fever vaccine virus transmitted via breastmilk. We describe a second case in which the vaccine virus was possibly the etiologic agent of meningoencephalitis. The Brazilian Ministry of Health now recommends delaying vaccination of nursing mothers until their children reach 6 months or providing them with guidance on alternative options to avoid the risk of transmission of the vaccine virus via breastmilk.J Pediatr (Rio J). 2011;87(3):269-272