Annually, acute bronchiolitis (AB) occurrence peaks during winter and is probably associated with air pollution. Aim: To relate the number of ambulatory consultations, emergency and hospital admission due to AB with climatic factors and air pollution. Patients and methods: Patients of less than 1 year old with AB that consulted to outpatient clinics, the emergency room or were admitted to the Pediatrics ward of the Catholic University Hospital, were enrolled. Information about respiratory syncytial virus (RSV) was obtained from the Catholic University Medical Investigation Center. Indices of air pollution such as particulate matters of less than 10 microns/m³ (PM 10), of less than 2.5 microns/m3 (PM 2.5), CO, SO3 and O3 were obtained from the Metropolitan Environmental Service. Temperature, humidity and precipitations were obtained from the Chilean Meteorological Service. Results: Ninety nine consultations in out patient clinics and 442 in emergency rooms were collected (55% male, mean age 4.8 months). One hundred fifty two were admitted (34.4%). Thirty percent of children consulting in emergency rooms were younger than 3 months and 43% of them were hospitalized. The RSV study was made in 307 patients and 52% were positive. There was a higher rate of hospital admissions among RSV positive than RSV negative patients (52.5 and 22% respectively, p<0.001). No association between environmental variables or air pollution and the number of consultations was observed. Young age and smoking inside the household were the main risk factors for hospital admission due to acute bronchiolitis. Conclusions: Environmental variables did not influence the number of cases of acute brochiolitis. Young age and exposure to tobacco smoke were risk factors for hospital admission (Rev Méd Chile 2003; 131: 1117-22).