Three thousand children aged 18 months were screened for dental caries and ongoing breast-feeding at 46 child welfare centres in different parts of Sweden. Of these, 200 children were selected for a more comprehensive examination, involving investigation of dietary, toothbrushing and sucking habits, use of fluoride, and determination of salivary levels of mutans streptococci and lactobacilli. The children were divided into four groups: group 1: children with caries not being breast-fed; group 2: children with caries being breast-fed; group 3: caries-free children being breast-fed; group 4: caries-free gender- and age-matched reference children not being breast-fed. The results showed that 63 of the children (2.1%) had caries and 61 (2.0%) were still being breast-fed. Twelve (19.7%) of the 61 children still being breast-fed had caries compared with 51 (1.7%) of the 2939 children not being breast-fed; the difference was statistically significant. Children with caries and still being breast-fed had a mean defs of 5.3, and those with caries not being breast-fed 4.9; the difference was not statistically significant. Children with caries, irrespective of whether they were being breast-fed or not, had significantly higher numbers of cariogenic food intakes per day than caries-free children. Mutans streptococci were detected in 67% of the children and lactobacilli in 13%. Children with detectable mutans streptococci and lactobacilli had significantly more caries than those without. The results indicate that Swedish children with prolonged breast-feeding have a tendency to establish unsuitable dietary habits which constitutes a risk situation for developing caries at an early age.