OBJECTIVES: The purpose of this study was to describe physiologic and behavioral pain behaviors in postoperative critically ill infants. A secondary aim was to identify how these pain responses vary over time. DESIGN: This observational study was conducted in the pediatric intensive care unit at two tertiary referral hospitals. Using ethological methods of observation, video recordings of postoperative infants were viewed to depict different situations of pain and no pain and were then coded using a reliable checklist. PATIENTS: A total of 803 recorded segments were generated from recordings of five critically ill infants aged between 0 and 9 months who had undergone major surgery. MEASUREMENTS AND MAIN RESULTS: There was an 82% agreement between the two coders. Multivariate analyses showed that physiologic responses differed only when adjusted for time. Significant decreases in systolic and diastolic arterial pressure (p < .001 and p = .036, respectively) were associated with postoperative pain exacerbated by painful procedures on day 2. On day 3, however, heart rate, arterial pressure (systolic, diastolic, and mean), and central venous pressure significantly increased (p < .05) in response to postoperative pain. Indicators included vertical stretch of the mouth, hand twitching, and jerky leg movements for postoperative pain and increase in respiratory distress, frown, eyes tightly closed, angular stretch of the mouth, silent or weak cry, jerky head movements, fist, pulling knees up, and spreading feet for postoperative pain exacerbated by painful stimuli. CONCLUSIONS: Findings support the ability to capture different intensities of postoperative pain in critically ill infants beyond neonatal age. These pain indicators can be used for the development of a pain assessment tool for this group of infants.