Background: Since the mid-1990s, there has been an increase in early childhood caries (ECC) in Australia and an increase in children living in one-parent families.Objective- To examine whether single parenthood (mother only) affects the effectiveness of an oral health promotion programme to prevent ECC in their child. Methods: First-time mothers were enrolled in a randomized controlled trial of anticipatory guidance to prevent ECC. The intervention was applied during pregnancy and when the child was 6 and 12 months old. Mothers in the control group received no intervention. The presence of ECC was assessed at 20 months of age and compared between children from one- and two-parent families. Results: Of 649 women enrolled, 441 brought their child for dental assessment. Eighty-seven (19.7%) had a one-parent family. Children from one-parent families had a 2.3 times higher incidence of ECC than children from two-parent families. The intervention reduced the frequency of ECC from 8.1% to 1.1% in two-parent families (relative risk: 0.14) and from 16.3% to 4.5% (relative risk: 0.28) in one-parent families. One case of ECC was prevented for every nine single mothers receiving anticipatory guidance compared with one case per 15 partnered mothers. Despite a greater reduction in the absolute risk of ECC in children from one-parent families, the intervention reduced their ECC experience only 3.5-fold compared with sevenfold in children from two-parent families. Conclusion: The intervention produced a greater reduction in the frequency of ECC in children from one-parent families than in those from two-parent families. This did not reduce their disadvantage, though, as they still had a four times higher risk than children from two-parent families. Mothers and children in one-parent families need substantially more attention and support than those in two-parent families to eliminate their disadvantage in suffering ECC.