BACKGROUND:Whilst the global burden of caries is increasing, the trajectory of decay in young children and the point at which prevention should occur has not been well established. AIM:To identify the 'natural history' of dental caries in early childhood. DESIGN:A birth cohort study was established with 467 mother/child dyads followed at 1, 6, 12, 18, and 36 months of age. Parent-completed surveys captured demographic, social, and behavioural data, and oral examinations provided clinical and data. RESULTS:Eight per cent of children (95% confidence interval (CI): 5-12%) at 18 months and 23% (95% CI: 18-28%) at 36 months experienced decay. Interesting lesion behaviour was found between 18 and 36 months, with rapid development of new lesions on sound teeth (70% of teeth, 95% CI: 63-76%) and regression of many lesions from non-cavitated lesions to sound (23% of teeth, 95% CI: 17-30%). Significant associations were found between soft drink consumption and lesion progression. CONCLUSIONS:Findings suggest optimal time periods for screening and prevention of a disease which significantly impacts multiple health and well-being outcomes across the life course.