Rotavirus is the leading cause of severe acute gastroenteritis among infants and young children, accounting for an estimated 527,000 deaths among children aged <5 years worldwide in 2004 (1,2). In the United States, rotavirus causes few deaths (20-60) each year, but remains a substantial cause of morbidity among children, resulting in approximately 55,000--70,000 hospitalizations, 205,000-272,000 emergency department (ED) visits, and 410,000 physician office visits. In the continental United States, rotavirus activity follows a distinct winter-spring seasonal pattern. In winter months, approximately 50% of hospitalizations and ED visits and 30% of outpatient visits for acute gastroenteritis among U.S. children aged <3 years are caused by rotavirus. To prevent rotavirus disease, in February 2006, a human-bovine rotavirus vaccine, RotaTeq (Merck & Co., Inc., Whitehouse Station, New Jersey), was recommended for routine use among U.S. infants . To summarize rotavirus activity through May 3, during the current 2007-08 season, CDC analyzed data from the National Respiratory and Enteric Virus Surveillance System (NREVSS) and the New Vaccine Surveillance Network (NVSN). The results indicated that, when compared with the 15 previous seasons spanning 1991-2006, rotavirus activity during the current season appeared delayed in onset by 2-4 months and diminished in magnitude by >50%. Additional surveillance and epidemiologic studies are needed to confirm the impact of rotavirus vaccination on the 2007-08 season and to monitor the impact of the vaccine on the incidence and epidemiology of rotavirus during future seasons.