To further define the clinical features and natural history of unsuspected Haemophilus influenzae type b (Hib) bacteremia, we retrospectively reviewed the records of 322 Hib infections observed during a 45-month period at Children's Hospital, Boston. We identified 31 patients with unsuspected Hib bacteremia and 19 with unsuspected Hib antigenemia and sterile blood cultures. Bacteremic patients were typically under two years of age (81%), had high fevers (mean = 39.5 degrees C), and frequently had otitis media (65%) diagnosed as their only focus of infection at presentation. Nineteen of 31 were empirically treated with oral antibiotics. Ten of 31 (32%) developed focal infectious complications consisting of meningitis (n = 7), cellulitis (n = 2), and pneumonia (n = 1). Children with focal infectious complications differed from those without infectious complications in having a significantly higher mean fever of 40.3 degrees C compared to 39.7 degrees C (P less than 0.05). Five of 19 (26%) empirically treated patients developed focal complications (all meningitis) compared to five of 12 (42%) untreated patients. Blood cultures at follow-up visit were positive in three of 19 (9%) treated patients and seven of 12 (42%) untreated patients (P less than 0.05). Of the 19 children with antigenemia and sterile blood cultures, 16 (84%) were empirically treated with antibiotics, and none had positive blood cultures or focal infections on follow-up evaluation. Children with occult Hib bacteremia are at high risk for developing serious focal infections, particularly meningitis, despite empiric antibiotic therapy. Once Hib bacteremia is suspected, strong consideration should be given to parenteral in hospital antibiotic therapy. The utility of rapid antigen detection for identifying high-risk patients requires further evaluation.