Human cytomegalovirus is the most common cause of congenital and perinatal infections throughout the world. Primary infection with human cytomegalovirus usually follows a benign course, but the virus remains latent or persistent in the host cell thereafter. Understanding the epidemiology of human cytomegalovirus is a key element in the development of strategies for prevention of infection. Although the actual sites of latency or persistence of human cytomegalovirus infections are still controversial, peripheral blood mononuclear cells and endothelial cells appear to be major sites of infection. Persistent infections caused by human cytomegalovirus could be augmented by a decrease in major histocompatibility complex expression as well as by virus-mediated immune dysfunction. It is possible that specific cellular interactions as well as production of several cytokines are necessary for the reactivation of human cytomegalovirus. Breast-fed infants are susceptible to human cytomegalovirus infection from breast milk. Human cytomegalovirus was isolated more frequently from breast milk at more than 1 month after delivery than from colostrum or early breast milk. Human cytomegalovirus DNA was also not detected in colostrum, but was found in breast milk samples 1 month after delivery. To clarify the role of milk cells and whey in vertical infection by breast feeding, we separated breast milk into milk cells and whey and examined each fraction. Human cytomegalovirus was isolated more frequently from milk whey samples than from cell samples. Human cytomegalovirus particle shedding into whey may be more important in vertical infection by breast milk than cell-to-cell transmission. The supernatant of colostrum did not exert an inhibitory effect on human cytomegalovirus-infected cells. Serum levels of cell free soluble interleukin-2 receptor of mothers with DNA-positive milk at 1 month after delivery were significantly higher than those of mothers with DNA-negative milk. It is likely that levels of factors such as soluble interleukin-2 receptor in serum are related to the reactivation of human cytomegalovirus which occurs locally in the mammary gland of the lactating mother after delivery. This minireview focuses on recent advances in the study of human cytomegalovirus infection of breast milk.