Immunization against pertussis was introduced in Sweden in the 1950s and discontinued in 1979. This was followed by a low endemic level of pertussis for 3 years. Thereafter the incidence gradually increased and there were two outbreaks in 1983 and in 1985. In the period 1980 to 1985 pertussis was confirmed by culture or serology in 36,729 patients of which 11% were younger than 12 months of age and 69% were ages 1 to 6 years. An estimate of the total frequency of pertussis in preschool children was made from reports from a sample of the child health centers. The annual incidence rate per 100,000 population ages 0 to 6 years increased from the 700 cases in 1981 to 3200 in 1985. The ratio of total cases to those reported from the laboratories was 3:1 in 1981 and 2:1 in 1985. The cumulative incidence rate by the average age of 4 years was estimated at 16% of the unimmunized cohort born in 1980 compared with 5% of the immunized cohort born in 1978. The seriousness of pertussis was evaluated by studying the 2282 pertussis patients hospitalized from 1981 to the end of 1983. Forty-eight percent were infants younger than 12 months of age. Neurologic complications were noted in 4% and pneumonia in 14% of the hospitalized patients. Eleven children received assisted ventilation. Fatal outcomes were reported in 3 children (0.1%), 2 of whom had severe congenital disabilities.