INTRODUCTIONDiagnostic and/or therapeutic procedures are required for the survival of critically ill newborn babies, and the possible pain generated by such procedures has been a source of concern. Simons et al.1 studied 151 neonates during their fi rst 14 days of neonatal intensive care unit (NICU) stay and found that each of them was subjected to an average of 14 painful procedures per day. Prestes et al.2 evaluated four university neonatal units in São Paulo for one month in 2001 and found that an average of three to fi ve potentially painful procedures were performed every day.Pain relief measures are hardly ever used for neonates that are subjected to such potentially painful procedures. Specifi c analgesic or anesthetic treatment is utilized for only 3% of newborn infants undergoing painful procedures, while nonspecifi c analgesia is given to 30% of them for reasons other than the procedures per se.3 According to Simons et al., 1 15-32% of 1,375 patients-day in NICU during their fi rst two weeks of life received some dose of analgesia every day. These data are concordant with those of Prestes et al., 2 who found that, out of 1,025 patients-day in NICU, 23% received at least one dose of analgesics.The most frequently cited reasons for pain undertreatment are the various myths surrounding painful experiences in neonatal populations: the likely incapacity of newborn infants to feel and express their pain, the diffi culty in measuring the painful phenomenon in pre-verbal infants and the insuffi cient availability of effective and safe therapeutic options for pain treatment. However, among these reasons, the gap between scientifi c knowledge and practical management is mainly due to the diffi culty of pain assessment in pre-verbal infants. 4 Pain evaluation in neonatal populations is not an easy task: the subjective nature of the painful experience and the existence of few reliable and valid evaluation instruments with clinic applicability, for measuring pain presence and intensity are diffi cult barriers to surmount. [5][6][7] Moreover, especially with regard to premature infants at different phases of their central nervous system growth and development, the responses to repetitive pain may change over time, thereby making evaluation and treatment of pain very diffi cult. 8 OBJECTIVEThe objectives of this study were to analyze pain expression among preterm newborn infants following repetitive pain stimuli and to investigate whether physiological and/or behavioral pain is amplifi ed or habituated during the fi rst week of life. METHODSThis prospective study included preterm newborn infants admitted to the NICU of São Paulo Hospital, Universidade Federal de São Paulo -Escola Paulista de Medicina, from January to October 2003, following approval by the Research Ethics Committee of Universidade Federal de São Paulo. Written consent from the mother was required before patients were included in the study.These premature infants included had gestational ages greater than or equal to 24 weeks and less than...