In vitro, lactoferrin (LF) strongly inhibits human cytomegalovirus (HCMV), which led us to hypothesize that in vivo HCMV might also be inhibited in secretions with high LF concentrations. In breast milk, high viral loads observed as high viral DNA titers tended to coincide with higher LF levels. However, the LF levels did not correlate to virus transmission to preterm infants. The viral load in the transmitting group was highest compared to the nontransmitting group. We conclude that viral load in breast milk is an important factor for transmission of the virus.