To estimate the association between behavioral and physiological pain measures and to identify determinants predicting the level of association, the COMFORT 'behavior' scale, heart rate (HR), mean arterial pressure (MAP), and the variability of HR and MAP (HRV and MAPV) were assessed every 3 hours after major abdominal or thoracic surgery. Subjects were 204 infants aged 0-3 years. The within-subject correlations, using the repeated measures, were 0.37, 0.44, 0.48, and 0.49 for COMFORT 'behavior' with HRV, HR, MAP, and MAPV, respectively. Neonates had lower behavior-physiology correlations than the older infants, due to low pain scores. Pain characteristics significantly predicted the COMFORT 'behavior'-HR/MAP correlations, suggesting that the behavior-physiology correlations increase with increasing pain. The behavior-physiology correlations were not greatly affected by physical condition. These data demonstrate large interindividual differences in behavior-physiology correlations after major surgery in 0- to 3-year-old infants. These differences should be further explored in future research.