A case-control study was used to assess whether inadequate exposure to the immunological benefits of human milk may affect infants' response to infection and make them more susceptible to childhood malignancies. 201 Denver children with cancer diagnosed at 1.5-15 years of age were compared with 181 controls, who were selected to be similar to cases for age, sex, and area of residence. Infant feeding categories were: breast feeding (BF) greater than 6 months; BF less than or equal to 6 months; and artificial feeding (AF, or exclusive non-human milk feeding). Compared with BF greater than 6 months, a raised risk for total cancers was found in both BF less than or equal to 6 month and AF groups. This increased risk was largely due to an increased incidence of lymphoma (n = 26).