In the United States, the prevalence of obesity increased by 30% in the decade from 1980 to 1990, and this increase appears to be continuing. Although obesity is recognized as a disease of multiple causes, a virus infection as an etiologic factor has been ignored until now. Five different viruses have been shown to cause obesity in animal models. Canine distemper virus was the first virus reported to cause obesity in mice. This report was followed by a report of Rous-associated virus-7, an avian virus, causing stunting, obesity, and hyperlipidemia in chickens. Next, Borna disease virus, a RNA virus of horses and sheep, was shown to cause obesity in rats. The next two reports are of adenoviruses, SMAM-1, an avian adenovirus that caused obesity in chickens, and Ad-36, a human adenovirus that caused obesity in chickens and mice. Association with human obesity was the unique feature of SMAM-1 and Ad-36. Ad-36 is the only human adenovirus to be implicated in obesity. Although the exact mechanism of virus-induced obesity is unclear, involvement of some viruses in the etiology of obesity urges us to consider a role for other pathogens in etiology of obesity. (C) 2000 Lippincott Williams and Wilkins, Inc.