Mycoplasma hominis (M. hominis) is a well recognized extragenital pathogen. Its role as a pathogen of the respiratory tract, however, remains difficult to determine. Four cases of pneumonia are presented in which M. hominis was isolated from blood, tracheal aspirates or post-mortem lung tissue. All 4 of these isolates were in young Aboriginal adult males. M. hominis is the least fastidious of the human mycoplasmas and grows well on most blood-containing bacteriological media. The recognition and identification of M. hominis is important as it is commonly resistant to erythromycin which would be the drug of choice in most atypical pneumonias. Early recognition and treatment with tetracycline instead may influence outcome.