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An outbreak of pertussis from July, 1993, to April, 1994, in Chicago was investigated to identify potential contributing factors.Surveillance was enhanced to identify cases. Information from a vaccination coverage survey was used to define a retrospective cohort to estimate vaccine effectiveness of three or more doses of pertussis vaccine.The median age of 218 reported cases was 8 months, 46% had Hispanic surnames and cases were clustered geographically. Vaccination status was known for 173 of 191 (91%) children younger than 6 years of age. Of these 173,90 (52%) were younger than 7 months, and 35 (20%) children at least 7 months of age had received fewer than 3 doses of pertussis vaccine. Pertussis vaccine effectiveness was 76% (95% confidence interval, 29 to 92).The limited ability of the current pertussis vaccination schedule to protect young infants accounted for 52% of cases, primary vaccine failure accounted for 28% of cases and failure to vaccinate children on time accounted for 20% of cases in young children. Low vaccine effectiveness did not appear to be a contributing factor.