To characterize the morbidity of pertussis in Canada, we did a retrospective review of all children with laboratory-confirmed pertussis seen at the Hospital for Sick Children, Toronto, between 1980 and 1990. A total of 975 patients were identified, of which 223 (23%) were admitted to hospital. The peak incidence of disease was observed in the fall. The incidence of disease did not vary with gender. Only 41% of children admitted had the classical symptoms of pertussis (paroxysmal cough and whoop). Compared with children older than 6 months of age, children younger than 6 months of age were more likely to be hospitalized, tended to be hospitalized longer, were less likely to be age-appropriately vaccinated and were more likely to require intensive care unit monitoring. Seventeen (8%) of 223 children required intensive care unit monitoring, and 12 of these children required mechanical ventilation therapy, for a duration of 3.5 +/- 0.6 days (mean +/- SD). One (0.1%) patient with secondary bacterial pneumonia died. This hospital-based study indicates that pertussis continues to be a cause of serious illness in children, particularly those younger than 6 months of age.