Serious bacterial infections occurred in ten children (1.4%) of 710 patients with croup admitted to the Soroka Medical Center during the years 1983-1989. Sixty-four patients (9% of all croup patients) were admitted to the pediatric intensive care unit (PICU), and 13 of them (20%) required intubation. Bacterial infections were noted in nine of the 13 intubated patients, in none of the other 51 PICU patients who did not require intubation and in one of the 646 patients (0.2%) who were not admitted to the PICU (p less than 0.0001). There was no difference in age, ethnic origin, or body temperature on arrival between the two PICU groups. Causative microorganisms were isolated from blood samples (three cases) and tracheal pus (eight cases). All intubated PICU patients were seriously ill: eight had bacterial tracheitis and one supraglottitis. Patients with bacterial tracheitis required frequent suctioning of the trachea for copious purulent secretions. The single patient with bacterial infection who was not admitted to the PICU had transient bacteremia. We conclude that the need for intubation in croup patients was an indicator for the presence of a serious bacterial infection.