Study Purpose: To investigate the effects of early-life vitamin D supplementation on the dental health of New Zealand (NZ) children. Background: NZ has a high caries burden, where dental disparities exist from a very young age. Children of Māori and Pacific ethnicity have a higher risk of developing early childhood caries (ECC) compared to NZ European children, as do children living in socioeconomically deprived households. Vitamin D deficiency (VDD) has been linked with ECC, particularly if it is present during tooth developmental periods. VDD is prevalent in pregnant NZ women and young Māori and Pacific children. Aim: To assess the effects of pregnancy and infancy vitamin D supplementation on ECC and developmental dental defects. Hypothesis: Pregnancy and infancy vitamin D supplementation protects the primary dentition against ECC and enamel developmental defects. Objectives: To compare the dental health of preschool children, who received one of two vitamin D supplementation doses or placebo, during pregnancy and infancy. Methodology: Participants were recruited from the Pregnancy and Infancy Vitamin D (PIVID) study, where 260 pregnant women and their infants were randomly assigned to one of three treatment groups. Vitamin D or placebo supplementation was given to pregnant women from 28 weeks gestation until delivery, and their infants from birth to age six months. The three supplementation groups comprised of: higher dose vitamin D, lower dose vitamin D (mothers: 2000/1000 UI, children: 800/400 IU per day, respectively), and placebo (Grant et al., 2014). The study sample comprised of approximately 30% Māori and 50% Pacific children. Children from the PIVID study were dentally examined at ages two, three and four. Data were collected on dental outcomes, demographics, and maternal and infant dental risk factors. Descriptive statistics and regression models were used to compare the effects of vitamin D supplementation on childhood dental health outcomes. These were: the presence of all 20 primary teeth, decayed missing and filled teeth (dmft), enamel developmental defects, dental plaque present and plaque and gingival indices. Results: This observational study followed a cohort of children previously enrolled in the PIVID randomised control trial (RCT). This study was not powered to be able to show statistical significance for clinically important differences in oral health outcomes between study groups. Children in the higher dose vitamin D group had odds of 3.16 (95% CI 0.90-14.78, p = 0.096) of having all 20 primary teeth present at age two, compared to placebo, and odds of 0.39 (95% CI 0.13-1.11, p = 0.081) of having decayed teeth at three years old. Children in the higher dose vitamin D group had increased odds of dental plaque present on teeth, at ages two (OR = 7.29, 95% CI 1.87-48.52, p = 0.012) and three (OR = 4.20, 95% CI 1.27-19.79, p = 0.032), compared with placebo. Additional risk factors associated with increased odds for ECC were irregular dental visits, ethnicity, and a higher intake of sugar snacks. Conclusion: In NZ, where ECC and vitamin D deficiency are prevalent, vitamin D supplementation in pregnancy and early infancy may be beneficial in protecting the primary dentition against ECC.