The incidence of acute viral respiratory diseases (confirmed by immunofluorescence test) was compared in a group of 159 infants aged 0-9 months breast-fed at least once daily and in a group of 608 infants of similar age and hospitalization duration receiving only artificial food. The incidence was significantly lower in breast-fed infants (p = 0.00057), especially in the first trimester of life (p = 0.00039). This was valid of all RSV infections (p = 0.01854) and parainfluenza type 4 virus infections (p = 0.0846). The antibody level against parainfluenza type 1 and 3 (HI) and against RSV in IgA class (ELISA) was compared with that in mother's milk and serum. No significant correlation was found for parainfluenza type 1 antibodies (r = 0.159) and 3 (r = 0.025), while for RSV the correlation was positive and statistically significant though rather low (r = 0.234). No significant correlation was found between milk antibody levels and the incidence of acute viral respiratory infections. Breast-feeding showed a significant influence on the lower incidence of these infections, particularly in the first trimester of life. It seems that this was associated with a non-specific mechanism rather than with the milk levels of specific antibodies.