We selected 327 7-month-old infants and divided them into two groups based on the frequency of salivary close contacts between mother and infant. Five to seven years later, all first-born children (N = 55) whose dental development had been followed regularly, were examined for dental caries and prevalence of salivary mutans streptococci (MS) and lactobacilli. The children with frequent maternal close contacts (F group, N = 21) had significantly less MS in saliva than the children with rare close contacts (R group, N = 34, P = 0.02). Only 19% of the children in F group compared with 56% in R group had experienced caries in their primary molars and/or canines (P < 0.01). A significantly greater proportion of the children in F group (57%) than in R group (27%, P < 0.05) had a high intake of sugar-containing foods and drinks in a 2-day dietary history. The F and R groups did not differ significantly with respect to other children's caries risk factors, or in age, sex, stage of dental development, dental treatment, or the social aspects studied. There were no significant differences between F and R groups in maternal caries experience, salivary MS or lactobacillus counts, or in maternal background factors (age, breast feeding, or education). Frequent transfer of maternal saliva to the mouth of the baby before tooth eruption was negatively associated with oral infection by MS and to caries in the primary dentition, possibly due to protective immune mechanisms.