Each year in the United States, an estimated 75,000-125,000 hospitalizations related to respiratory syncytial virus (RSV)occur among children aged <1 year, and RSV infection results in approximately 1.5 million outpatient visits among children aged <5 years. In the United States, RSV season begins in the fall, peaks in winter, and ends in the late winter and early spring. However, the exact timing and duration vary from year to year and by geographic region. To describe trends in RSV seasonality, data from the National Respiratory and Enteric Virus Surveillance System (NREVSS) were used to determine the onset, offset, and peak of the July 2010--June 2011 RSV season, and for an aggregate analysis of the four most recent RSV seasons (July 2007--June 2011). During 2010--11, excluding Florida, season onset occurred from mid-November to early January, and offset occurred from mid-March to late April across all 10 U.S. Department of Health and Human Services (HHS) regions. Florida is reported separately because it has an earlier onset and longer duration than the rest of the country. During the four seasons from 2007 through 2011, onset among the HHS regions excluding Florida ranged from mid-October to early January, and offset ranged from early February to early May. Information on national and regional patterns can be used by clinicians and public health officials to guide diagnostic testing during respiratory disease outbreaks and determine when to provide RSV immunoprophylaxis for children at high risk for serious complications.