Objective: To assess the efficacy of Kangaroo Care (KC) in decreasing stress in newborns of 29-34 weeks' post-menstrual age (PMA). Methods: Quasi-experimental pre-post without control group analytical study conducted in the Gregorio Maranon University Hospital Neonatal Unit. The inclusion criteria were: infants 29 to 34 weeks' PMA, in an incubator, did not have neurological pathology, were not post-surgical, and with a Spanish-speaking mother and/or father. The sample size was fifty-one premature infants. The study variables selected were: clinical variables (additional oxygen and pathology), socio-demographical variables (PMA, KC duration) and the outcome variable, premature infant stress, which consisted of two variables: the physiological stress signal and the behavioural stress response. The variables were collected at three different times: basal stress, during KC and after KC completion, making a comparison analysis between the basal stress and after KC. Results: The response rate was 100%, without registering any loss. The stress variables that changed after the intervention (statistically significant) were: irregular breathing, trunk arching or hyperextension, very open fingers, contraction of the face muscles, apnea, irritability and exaggerated and sustained extension of arms and legs. O-2 saturation was 94.73%+/- 3.05% before KC and 95.92%+/- 2.97% after the intervention. The heart rate (HR) ranged from 158.14 +/- 17.48 bpm (beats per minute) before the KC to 151.47 +/- 4.47 bpm after it. Conclusions: KC is related to the decrease in the occurrence of neonatal variables of stress, helping to organize motor and physiological systems to achieve a state of tranquillity. (C) 2010 Elsevier Espana, S.L. All rights reserved.