It has been hypothesized that non-human milk feeding may increase the risk for cancer or for a specific cancer or group of cancers as well as the risk for diseases such as type-1 diabetes mellitus and Crohn's disease. Regarding DNA damage leading to cancer development in the absence of human milk protection, a comparison between infants fed human milk and cow's milk has been performed. Each group consisted of 35 infants, whose ages ranged from 9 to 12 months. The level of DNA damage in the peripheral blood lymphocytes of infants has been studied by the comet assay. A significant increase has been found in the number of limited DNA-damaged (p < 0.001) and extensive DNA-damaged (p < 0.001) cells of infants fed cow's milk. To our knowledge, this is the first study using the comet assay on infants not breastfed. Supporting our previous SCE study, these results suggest that there is some level of DNA damage in the lymphocytes of infants not breast-fed and this may lead to malignancy in childhood or later in life. Copyright (C) 2003 S. Karger AG, Basel.