The roles of Mycoplasma pneumoniae, M. hominis and Ureaplasma urealyticum in diseases of humans are currently under investigation. M. pneumoniae, which causes primary atypical pneumonia, is a well established pathogen of the respiratory tract. Complications of infection by this organism are also being recognized; they include disorders of the hematopoietic, cardiovascular, central nervous, musculoskeletal, cutaneous and gastrointestinal systems. The roles of the genital mycoplasmas M. hominis and U. urealyticum are controversial but may include infections of the genitourinary tract and in pregnancy as well as diseases of the newborn, such as neonatal pneumonia and meningitis. In this review atypical pneumonia due to M. pneumoniae is described and the role of mycoplasmas in other diseases is discussed.