Respiratory Syncytial Virus (RSV) is a frequent cause of hospital admission in young children and high risk babies such as premature newborns, or babies with underlying cardiac or pulmonary disease, or immunodeficiency. Outbreaks occur most frequently in the cold season in areas with temperate and Mediterranean climates. Aim of the "Osservatorio VRS" Study was to describe the time-related pattern of RSV epidemics in Italy, across four consecutive epidemics, from 2000 to 2004. Nasal specimens for RSV detection were obtained and tested by an immunoenzymatic test. A total of 2110 children were tested for RSV determination, the rate of children with RSV infection was 21%, and that of children hospitalized for RSV disease was 49%. Considering the whole study period, the RSV epidemics started in October-November and ended in May, showing a peak incidence in February, with a median of 28.1% and a maximum of 48.9%. Analysis of monthly distribution of each year of the study showed a biennial trend for an earlier appearance. A different epidemiological pattern of the infection was observed among the three national areas. In conclusion, even though the mechanism governing RSV infection periodicity remains unknown, its awareness in the absence of an RSV surveillance system as in Italy, may be useful for scheduling RSV prophylaxis and for hospital resource management.