The aim was to study whether 3-yr-old children's toothbrushing habits are associated with the child's other dental health habits and whether the mother's background influences the child's toothbrushing frequency. The survey used stratified randomized cluster sampling, confidential questionnaires and clinical dental examinations. A random sample of 1443 (91.2%) of a Finnish province's 1582 primiparous women participated in the study at the onset of their pregnancy. Dental health care clinics of the public health care system carried out dental examinations in 1018 (83.5%) 3-yr-old children born of these pregnancies. The variables used in the study included consumption of juice at night and sugar at the age of 1.5 yr and the use of fluoride tablets and sweets at the age of 3. The mother's background factors included age, basic education and occupation. Daily toothbrushing was practised by 78.2% of the children. Addition of sugar to the diet and frequent use of sweets at the age 3 were more common in those who brushed their teeth only occasionally, whereas the use of fluoride tablets was less frequent in them than in those brushing their teeth every day. Of the mothers' background factors, age was the most significant. The youngest mothers paid the least attention to their children's toothbrushing habits. The proportion of those brushing their teeth was 67.9% in rural areas, 78.6% in semi-urban population centers and 80.1% in towns (P = 0.02). The results indicate that the health education provided by dental health care clinics should be focussed on young mothers and rural families.(ABSTRACT TRUNCATED AT 250 WORDS)