A matched and serologically confirmed case-control study was carried out to investigate the source of an outbreak of acute hepatitis involving 290,000 cases in the suburbs of Shanghai, in January 1988. A total of 132 patients with acute hepatitis from six different hospitals were chosen as cases and the same number of control patients without hepatitis were matched for gender, age, admission date and area of residence. Serum specimens from both case and control patients were detected for specific anti-hepatitis A (HA) IgM antibody and a questionnaire was used to investigate probable risk factors related to the outbreak. The positive rate of anti-HA IgM was 98.48% in the case group and only 0.76% in the control, indicating that the infection was caused by HA virus. The results revealed that the source and mode of transmission were due to the consumption of contaminated and inadequately cooked clams (Anadara subcrenata lischke). There was a highly positive dose-response relationship between the odds ratio of contracting HA and the quantity or frequency of clam consumption. The odds ratios of acquiring HA from clams were up to 62.4-63.4 by both group stratification and multiple unconditional logistic regression analyses.