In a large urban centre of a developed nation, 63 household contacts of 20 index cases with acute hepatitis A virus infection and 95 household contacts of 29 index cases with acute hepatitis B virus infection were prospectively followed for 2 years to document the risk of acquiring acute hepatitis from the index case. Twenty-one of 63 (33%) hepatitis A virus household contacts had serologic evidence of previous hepatitis A virus infection on the initial serum sample. Of the remaining 42 susceptible individuals, 22 (52%) were or became IgM anti-HAV positive within 6 months of the diagnosis in the index case. With respect to hepatitis B virus infection, 18/95 (17%) household contacts had serologic evidence of previous hepatitis B virus infection on the initial serum sample. Of the remaining 77 susceptible individuals, four (5%) had or developed serologic evidence of acute hepatitis B virus infection (IgM anti-hepatitis B core antigen positive) during the 2 years of follow up. In three of these four individuals, acquisition of hepatitis B virus was apparent within 6 months of the diagnosis in the index case. The results of this study indicate that in this urban centre, the risk of acquiring acute hepatitis A virus infection from index cases within the household is approximately 10 times greater than that for acute hepatitis B virus infection. These results support the need for continued passive and/or active immunization against hepatitis A and B virus infection in susceptible household contacts.