In late 2006, CDC began receiving requests from numerous state public health departments for information about a perceived increase in the number of outbreaks of acute gastroenteritis (AGE), especially those involving person-to-person transmission in long-term--care facilities. No national surveillance system exists for AGE outbreaks, including those caused by norovirus, unless foodborne transmission is suspected. In the absence of national surveillance data, CDC attempted to better characterize the outbreaks of AGE by analyzing information from the following sources: 1) detailed data on recent AGE outbreaks in three of the states that had contacted CDC about a possible increase (North Carolina, Wisconsin, and New York); 2) emergency department (ED) syndromic surveillance data from Boston, Massachusetts; 3) basic epidemiologic data on AGE outbreaks from a CDC survey of state health departments; and 4) laboratory data from CDC. The analysis suggests that a national increase has occurred in the frequency of AGE outbreaks caused by norovirus (including fatal cases in long-term-care facilities). Two new cocirculating GII.4 norovirus strains emerged nationwide in 2006 and likely accounted for this increase in activity. Improved national surveillance of outbreaks, including those with person-to-person transmission; development of accessible, affordable, and timely clinical tests; and increased access to a norovirus strain sequencing database at CDC will lead to more accurate assessment of the morbidity and mortality associated with norovirus and more rapid identification of newly emerging norovirus strains.