A 4-month-old female infant with meningitis caused by Haemophilus influenzae type f had a hospital course complicated by sterile subdural effusions and persistent neurologic abnormalities. One year later she was normal in all respects. The infant's mother had serum bactericidal antibodies to H. influenzae type b but not to type f. During recovery the patient had no bactericidal antibodies to type b, and the type f organism could not be maintained in her serum. Review of the literature identified 40 cases of meningitis reported as caused by H. influenzae other than type b. An evaluation of the ten cases described as due to encapsulated strains (a, e, and f) shows that the age distribution and clinical features are similar to those of meningitis caused by type b. Only five cases of meningitis caused by unencapsulated H. influenzae have been described. Four of the patients were older than the usual age range for type b meningitis and two had prior head trauma. A large clinical trial in Finland with a two-year observation period has demonstrated no untoward increase in non-b H. influenzae meningitis in recipients of a type b vaccine. Serious infections caused by other H. influenzae types will continue to occur sporadically and may increase in frequency when an effective vaccine against type b is widely used in infants.