During July 1988, 68 persons in Chattanooga, Tennessee, developed serologically confirmed hepatitis A. Between 15 June and 3 July, 93% of case-patients ate at a specific restaurant compared with only 3% of the local community. An intravenous drug user who worked as a cook was identified as the source. A case-control study was done to identify the vehicle of transmission. Case-patients were more likely than controls to have eaten hamburger buns and pickles, the only foods routinely handled after cooking. Of the restaurant patrons included in the study, 12 microwaved their food before consumption; none developed clinical illness despite eating large amounts of food handled after cooking. Sandwiches that were not microwaved were significantly associated with illness (odds ratio = 9.6; P less than .02). This epidemiologic evidence suggests that microwaves inactivate hepatitis A virus in food.