Oka/Merck varicella vaccine has been studied in this institution since 1981. Persistence of antibody for 6 to 8 years has been demonstrated; however, cases of chickenpox have been seen in immunized children. The severity of chickenpox in healthy children who have received Oka/Merck varicella vaccine since 1981 is described. All vaccinees who developed chickenpox-like rashes more than 6 weeks postimmunization were examined. Of 2163 vaccinees, 164 were examined, of whom 114 had rashes consistent with chickenpox. When sera were available (46%), antibody studies uniformly confirmed varicella-zoster virus infection. Chickenpox occurred 2 to 96 months (median of 44 months) postimmunization. The range for the number of skin lesions was 1 to 285 (median 18) in seroconverters. Symptoms included itching in 39%, fever in 9%, headaches in 7%, lymphadenopathy in 3%, and malaise in 2%; 54% were asymptomatic, except for the rash. The median time to total healing was 5 days. The median time lost from school was 2 days. Thirteen of the children in whom infections developed had failed to seroconvert after immunization. Their infections were similar in severity to those of children who had seroconverted originally. When varicella was introduced into families as a result of chickenpox in an immunized family member (index case), the rate of secondary chickenpox among immunized siblings was 12.2%. Eleven such secondary cases were similar in severity to the 9 index cases. It is concluded that chickenpox is generally mild in previously immunized children.