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Objectives. Pain exposure during early infancy affects the pain perception beyond infancy into childhood. The objective of this study was to examine whether major surgery within the first 3 months of life in combination with preemptive analgesia alters pain responses to immunization at 14 or 45 months and to assess whether these alterations are greater in toddlers with a larger number of negative hospital experiences. Methods. Two groups of 50 toddlers each were compared: index group and control group. All index toddlers had participated within the first 3 months of their life in a randomized, clinical trial that evaluated the efficacy of preemptive morphine administration for postoperative analgesia. The controls were matched by type of immunization and community health care pediatrician. Pain reactions were recorded at routine immunization at either 14 (measles-mumps-rubella immunization) or 45 months (diphtheria-tetanus-trivalent polio immunization) of age. Outcome measures were facial reaction, coded by the Maximum Discriminative Facial Movement Coding System; heart rate (HR); and cortisol saliva concentration. Negative hospital experiences included number of operations requiring postoperative morphine administration, cumulative Therapeutic Intervention Scoring System scores, and length of stay in the intensive care unit or total hospitalization days. Results. No differences were found between the index and control groups in the facial display of pain, anger, or sadness or in physiologic parameters such as HR and cortisol concentrations. Intragroup analyses of the index group showed that after measles-mumps-rubella vaccination, the number of negative hospital experiences correlated positively with the facial responsiveness and negatively with HR responses. No effect was seen after diphtheria-tetanus-trivalent polio immunization. Conclusions. Major surgery in combination with preemptive analgesia within the first months of life does not alter pain response to subsequent pain exposure in childhood. Greater exposure to early hospitalization influences the pain responses after prolonged time. These responses, however, diminish after a prolonged period of nonexposure. Pediatrics 2003;111:129-135; prospective study, newborn infant, repetitive pain, biobehavioral pain response, analgesia., It has been well established that the nociceptive pathways, (sub)cortical centers, and neurochemical systems necessary for pain perception and transmission are intact and functional from an early stage of fetal [...]