Mycoplasma genitalium is predominantly flask-shaped with cytadsorptive and glass-adhering properties. These biological features of pathogenicity are reflected in its ability to cause urethritis and salpingitis in subhuman primates when given experimentally, although its role in human disease is less clear. M. genitalium was initially recovered by culture of specimens from two of 13 men with acute nongonococcal urethritis (NGU), but culture remains a laborious and insensitive means of detection. This organism has, however, been detected in the urethra of > or = 20% of men with acute NGU by using a polymerase chain reaction (PCR) and in a similar proportion of men with chronic NGU. The possibility of sexual transmission of M. genitalium is supported by the fact that by using the PCR it has been detected in the lower genital tract of about 20% of women who were attending a clinic for sexually transmitted diseases. It has also been detected, together with Mycoplasma pneumoniae, in a joint of a hypogammaglobulinemic patient who was suffering from polyarthritis after pneumonia and has been isolated, again with Mycoplasma pneumoniae, from the throat in culture. Such detection raises the question of the preferred mucosal site of M. genitalium, which has yet to be determined.