Anaerobic meningitis occurred in four patients in whom anaerobic bacteria had not been suspected as a possible cause. The predisposing conditions were typical of those seen in patients previously reported to have this infection and included chronic otitis media with mastoiditis, chronic sinusitis, recent craniotomy and abdominal trauma. Two of the patients had undergone immunosuppression (immunosuppressed patients); a compromised immune system may facilitate the development of anaerobic meningitis in patients with the appropritate underlying conditions. Head and neck neoplasms, head trauma, suppurative pharyngitis and laminectomy wounds are additional situations in which anaerobic meningitis occurs. Anaerobic bacterial meningitis probably occurs more often than is recognized. The cerebrospinal fluid should be transported and cultured anaerobically when meningitis develops in a patient with a predisposing condition.