In order to demonstrate the occurrence of CMV perinatal infection in a middle socioeconomic class population, the authors conducted a 8-month prospective study in 37 children, not infected congenitally, born in a public hospital of São Paulo city, Prevalence of CMV-IgG antibodies in mothers, detected by immunoenzimatic assay (ELISA), was 92.7%. Survival analysis showed that the risk of acquiring CMV perinatal infection diagnosed by virus isolation in human fibroblasts was 30.9%. When the diagnostic method was detection of IgM class antibodies by indirect immunofluorescence the risk was 8.1% (p < 0.05). Milk samples inoculated in human fibroblasts failed to demonstrate the presence of virus. The infected children did not present any signal of disease in a 4-month follow-up.