Adult Proteus (P.) mirabilis meningitis is relatively rare and has not been examined individually in the English-language literature. During a period of 15 years (January 1986-December 2000), four adult patients with P. mirabilis meningitis and one adult patient with mixed bacterial meningitis involving P. mirabilis were identified at Chang Gung Memorial Hospital, Kaohsiung. These five patients included one man and four women, aged from 19 to 74 years (mean age=55.4). P. mirabilis infection accounted for 1.7% (4/229) of cases of our culture-proven monomicrobial adult bacterial meningitis and was involved in 7.1% (1/14) of cases of our adult mixed bacterial meningitis during this period. Underlying debilitating conditions including diabetes mellitus and neurosurgical disorders were common in these five cases. Adult P. mirabilis meningitis had an acute clinical course, with fever and consciousness-disturbance occurring as most prominent clinical manifestations in all patients. Other common manifestations included hydrocephalus, seizure, septic shock and wound infection. Hematogenous spread would appear to be the most likely mechanism. Multi-antibiotic resistant strains of P. mirabilis were not found in our patients. All strains were susceptible to third-generation cephalosporins, imipenem, aztreonam and ciprofloxacin. The results of treatment for adult P. mirabilis meningitis were not satisfactory, most of the patients surviving with severe neurological deficit.