Background: The aim of this study was (a) to statistically identify distinct trajectories of pain following surgery in infants less than 6 months of age, and (b) to compare these trajectories to descriptions of chronic pain in infants in the neonatal intensive care unit (NICU). Methods: This was a retrospective medical record review of infants admitted to a NICU between 2008 and 2018 following surgery. All infants who underwent one major procedure to the abdomen or thorax and returned to the NICU following surgery were included. Pain was assessed regularly using a validated Pain Assessment Tool. Group-based trajectory analysis was used to determine the trajectory of recovery from pain following surgery. Results: A total of 726 infants were included in the study. A two-group trajectory model, defined as typical and atypical pain trajectories, was selected. The typical group (n = 467) consisted of infants who had significantly fewer days (1.5 ± 2.3 vs 5.3 ± 5.5, p <.001) and recorded instances of pain (2.0 ± 3.4 vs 9.7 ± 10.5, p <.001) compared to infants in the atypical group (n = 259). The incidence of iatrogenic neonatal abstinence syndrome was greater in the atypical than the typical group (11% vs 5%, p =.001). Conclusions: This study has revealed two distinct pain trajectories in infants after surgery. While recovery from pain occurs within days in the typical group, the atypical group demonstrates pain for a significantly longer period, often weeks and months following surgery. This latter group adheres closely to current descriptions of chronic pain in infants.