ObjectiveTo analyse the influence of climatic factors on the number of hospitalised infants with respiratory syncytial virus (RSV) per week.MethodsA retrospective observational case–control study was designed enrolling infants under 2 years of age, admitted to hospital between October 1995 and June 2000 with lower respiratory tract infection due to RSV. Climatic and seasonal data were included. The week variable was used as the study unit: weeks with more than one admission for the case group and weeks without admissions for the control group. The total number of weeks excluding summer months, from June to September, was 174.ResultsA total of 167 infants were admitted to hospital with lower respiratory tract infection due to RSV with a peak in January and February. There was 82 weeks with one or more admissions (case group) and 92 without admissions (control group). The case group demonstrated lower levels of humidity (absolute: 5.6 ± 1.5 vs. 6.5 ± 1.5; p < 0.001) and lower temperature (ground level: 0.4 ± 3.2 vs. 2.2 ± 3.5; p < 0.001). When climatic factors were analysed in a logistic regression model, absolute humidity (p < 0.001) was an independent variable associated with a higher risk of infection.ConclusionsLow absolute humidity was independently associated with hospital admission of infants with lower respiratory tract infection due to RSV.