Objective: Previous research discerned 3 groups of caregiver-toddler dyads that differed in their physiological coregulatory patterns, also known as physiological attunement, during routine vaccinations in the second year of life. One group of dyads (80% of sample) displayed an attuned regulatory pattern, and 2 groups of dyads (20% of sample) showed maladaptive attunement patterns (ie, a lack of attunement or misattunement). The objective of the current study was to examine how well the pain-related distress of children and caregivers during vaccination predicted these patterns.Methods: Caregiver-toddler dyads (N = 189) were part of a longitudinal cohort observed at either 12-, 18-, or 24-month vaccination appointments. The caregiver's self-report of worry was assessed before and after the needle, and the child behavioral painrelated distress was also measured during the vaccination appointment. Logistic regression was used to determine how well these variables predicted caregiver-child physiological attunement patterns, as indexed by high-frequency heart rate variability.Results: Higher behavioral pain-related distress at various timepoints after the needle were associated with membership in the dyad groups that showed misattunement or lack of attunement. Further, caregivers with higher preneedle worry and lower postneedle worry had a greater likelihood of belonging to groups that showed a maladaptive attunement pattern.Discussion: Findings suggest that caregivers who experience distress associated with their toddlers' vaccination experience more difficulty coregulating with their child during vaccination, and these children are at risk of experiencing higher levels of pain-related distress. This research highlights the need to help caregivers support their children's regulation during vaccination.