Respiratory syncytial virus (RSV) is the most common cause of bronchiolitis and pneumonia in children aged <1 year and is a major cause of respiratory illness in older adults. RSV is transmitted person-to-person via close contact, droplets, and fomites. Each year in the United States, an estimated 75,000-125,000 children aged <1 year are hospitalized with RSV. Those at increased risk for hospitalization include premature infants meeting certain criteria and persons of any age with compromised respiratory, cardiac, and immune systems. RSV incidence follows a seasonal pattern. In temperate climates, the RSV season generally occurs during the fall, winter, and spring months. However, the timing of RSV circulation can vary by location and year. CDC analyzed laboratory data from the National Respiratory and Enteric Virus Surveillance System (NREVSS) to summarize RSV temporal and geographic trends in the United States during the weeks ending July 7, 2007--June 28, 2008, and for the first 5 months of the current reporting season (the weeks ending July 5-December 6, 2008). This report describes the results of that analysis, which indicated that the 2007-08 RSV season onset for the 10 U.S. Department of Health and Human Services (HHS) regions and Florida ranged from early July to mid-December, and the season offset ranged from late January to mid-April; the current 2008-09 season onset occurred in eight of the 10 HHS regions by December 6, 2008. These findings support previous observations that the RSV season not only varies by location, but can vary by year.