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Since the introduction of acellular pertussis vaccines in Japan late in 1981, more than 20 million doses have been administered, mostly to children 2 years of age and older. Clinical studies indicate that mild local and febrile reactions are less frequent after administration of acellular pertussis vaccines than after whole-cell vaccines. Serious adverse events with sequelae occurred in 2-year-old children at approximately the same low rate during the period 1975 through August 1981, when whole-cell vaccines were used, and during August 1981 through 1984, when acellular vaccines were used exclusively. Five household contact studies have yielded vaccine efficacy estimates ranging from 78% to 92% in children 1 year of age or older. In addition, there has been a continuing decrease in reported pertussis incidence from the epidemic peak in 1979. Additional data on the safety and efficacy of acellular pertussis vaccines administered to infants would be useful in consideration of acellular pertussis vaccine licensure in the United States. ( JAMA 1987;257:1351-1356)