A single unit of infected blood transfused into 11 neonates resulted in a large multistate outbreak of 55 cases of hepatitis A, 35 of which were symptomatic. The person who donated the blood to the 11 neonates became ill with hepatitis A one week after the donation. Hepatitis A infection was then acquired by nurses and physicians having direct contact with the neonates and by parents and relatives. Three additional newborns who had not received transfusions with the infected unit also acquired hepatitis A. The neonates with hepatitis A were all asymptomatic. The attack rate in susceptible nurses was 16%; in susceptible physicians, 4%. A survey of the relatives of the 11 newborns who received transfusions revealed eight symptomatic cases of hepatitis A in 32 immediate family contacts, ie, mother, father, and siblings, with a resulting attack rate of 25% uncorrected for prior immunity. The setting of the neonatal intensive care unit appears to be ideal for the transmission of hepatitis A.