Good evidence indicates the widespread carriage of U. urealyticum among sexually active females. Colonization of the upper genital tract appears to be a risk factor for adverse pregnancy outcomes, though this is not the case for lower-genital-tract colonization. Based on studies in which amniotic fluid obtained from amniocenteses was already positive for U. urealyticum, and its high prevalence in very-LBW neonates, it is likely that the infection is acquired during early pregnancy. A number of observational studies have linked this organism to pulmonary infections, meningeal infections, and bacteremias, particularly in LBW neonates. It is difficult, however, to separate the morbidity that is directly attributable to infection with the organism from morbidity owing to extreme prematurity. Problems in measuring the burden of illness resulting from this organism are compounded by difficulties in its diagnosis. The organism's fastidious nature prevents many laboratories from isolating it from specimens. Rapid and practical methods for identifying the organism are urgently needed. These need to be followed by RCTs to determine if outcomes of pregnant women and babies with various conditions, from whom the organism has been isolated, can be improved through treatment with antimicrobial agents.