BACKGROUND The Zika virus is an arbovirus that has as main source of transmission the bite of infected insects of the genus Aedes and has been associated with cases of congenital malformation and microcephaly in neonates. However, other sources of transmission have been identified since the emergence of this virus in the world population, such as vertical transmission by semen and possibly other body fluids such as vaginal secretion and breast milk. CASE REPORT An infant, born to a mother whose previous delivery was a baby with severe microcephaly, was normal and was negative for Zika virus at birth but developed secondary microcephaly 1 month later, that persisted. The baby was exclusively breast-fed and Zika virus was present in the mother's milk. CONCLUSIONS We report the detection of Zika virus exclusively in the breast milk of a woman after her second delivery of an infant, who later developed microcephaly. This case is consistent with possible vertical transmission.