Haemophilus influenzae meningitis is rare in adults. We reviewed the microbiological records of cerebrospinal fluid (CSF) and blood cultures and the medical records of patients with bacterial meningitis admitted to Chang Gung Memorial Hospital from January 1978 to May 1996. Haemophilus influenzae meningitis was found in six adult patients, accounting for 1.8% of 326 bacteriologically proven adult cases of meningitis diagnosed between January 1984 and May 1996. Predisposing conditions included head trauma and/or neurosurgical procedure in four patients and an extracranial extension of a pituitary adenoma in a diabetic patient. None of them had altered consciousness on the day of treatment. A CSF cell count less than 500 x 10(6) leukocytes/liter was noted in three patients. Antibiotic therapy was successful in all patients. Thus, predisposing conditions, particularly head trauma and/or neurosurgical procedure, are commonly present in adult patients with H. influenzae meningitis. Patients with the infection may have a low cellular count in the CSF that may be readily misinterpreted as meningitis caused by other pathogens.