Outbreaks of acute gastroenteritis are frequently caused by caliciviruses. Electron microscopy was used to search for these viruses in fecal samples from patients with acute gastroenteritis. Of 5800 samples collected and analyzed from November 1994 to June 1998, 3700 were associated with outbreaks. A total of 676 outbreaks were analyzed, and viruses were found in 67%. Caliciviruses, usually Norwalk-like viruses (NLVs), were found in 407 (89%) of 455 outbreaks, while Sapporo-like viruses were identified in nine outbreaks, including six that were suspected to include foodborne transmission. Sixty percent of the 1041 patients with calicivirus infections were between 70 and 90 years of age. Food- and waterborne infections were associated with 66 calicivirus outbreaks. Virus-positive outbreaks were documented mainly during winter and spring. The longitudinal survey showed that caliciviruses, and especially the NLVs, cause most nosocomial and community-associated outbreaks in Sweden.