Respiratory syncytial virus (RSV) causes lower respiratory infection among infants and young children worldwide. Annually in the United States, RSV infection has been associated with an estimated 57,527 hospitalizations and 2.1 million outpatient visits among children aged <5 years. In temperate climate zones, RSV generally circulates during the fall, winter, and spring. However, the exact timing and duration of RSV seasons vary by region and from year-to-year. Knowing the start of the RSV season in any given locality is important to health care providers and public health officials who use RSV seasonality data to guide diagnostic testing and the timing of RSV immunoprophylaxis for children at high risk for severe respiratory infection. To describe RSV seasonality (defined as onset, offset, peak, and duration) nationally, by U.S. Department of Health and Human Services (HHS) regions and for the state of Florida, CDC analyzes RSV laboratory detections reported to the National Respiratory and Enteric Virus Surveillance System (NREVSS). Florida is reported separately because it has an earlier season onset and longer season duration than the rest of the country. For 2012-13, the RSV season onset ranged from late October to late December, and season offset ranged from late December to late April, excluding Florida. For 2013-14, the RSV season onset ranged from late October to late January, and season offset from late January to early April, excluding Florida. Weekly updates of RSV national, regional, and state RSV trends are available from NREVSS at http://www.cdc.gov/surveillance/nrevss.