The increase in winter related health pediatric demand is associated with three factors: cold and rainy weather, air pollution and respiratory viral epidemics. During the winter of 2002 there was, successively, heavy rain, air pollution, cold weather and a respiratory syncytial virus (RSV) epidemic in Santiago. Aim: To study the influence of environmental factors and RSV epidemic on pediatric health care demand. Patients and methods: The number of hospital admissions and outpatient consultations for acute lower respiratory infections (ALRI) from April to October 2002 (Autumn to Spring in Chile), were registered in a public pediatric hospital of Santiago. A respiratory virus surveillance (RSV, adenovirus influenza and parainfluenza virus) was carried out among children admitted for ALRI. Climate conditions and air pollution (number of particles of more than 10 microns/mm3) values were provided by local environmental health services. Results: As expected, a rise in winter hospital admissions and outpatient consultations was detected, that peaked in week 29. This rise coincided with the higher RSV detection week. There was heavy rain in weeks 22, 23 and 30, as well as cold weather in June-July (weeks 23, 24, 26, 27 and 30; mid Winter), that did not increase health care demand. Likewise, high air pollution, registered from weeks 24 to 26, did not increase health care demand. Conclusions: RSV epidemic is the principal factor associated to the increase in health burden during winter in Santiago (Rev Méd Chile 2003; 131: 902-8)