Previous studies suggest that primary tooth eruption (PTE) in preterm infants is related primarily to gestational age, but the impact of other neonatal factors has not been studied. In a prospective longitudinal study, the timing and sequence of PTE were documented by a pictorial PTE record completed by the parents and by frequent oral exams in 14 preterm infants whose first tooth erupted at < or = 10 months chronologic age (normal group) and 21 preterm infants whose first tooth erupted at > 10 months (late group). Initial eruption sequence in both groups was the same as full-term infants, with the two lower central incisors erupting first. PTE occurred significantly later in children with BW < 1000 g (t = 3.4, P < 0.01) or < or = 30 weeks (t = 2.41, P < 0.05). Factors related to nutrition appeared to be important. Age at first tooth correlated significantly with age when full enteral feedings were attained, age when oral vitamin supplementation was started, and with average weight gain per day. Five neonatal factors (duration of oral intubation, birthweight, gestational age, age when full enteral feedings were attained, and apnea of prematurity) explained 44% (R = 0.67, P < 0.05) of the variability in age at which the first tooth erupted. Of that 44%, 77% was explained by a single factor, duration of oral intubation. These results suggest that factors related to severity of neonatal illness and postnatal nutrition as well as degree of prematurity affect timing of primary tooth eruption.