Ureaplasma urealyticum, a common commensal of the urogenital tract of sexually mature humans, is gaining recognition as an important opportunistic pathogen during pregnancy. While its etiologic significance in many aspects of adverse pregnancy remains controversial, recent evidence indicates that U. urealyticum in the absence of other organisms is a cause of chorioamnionitis. Furthermore, ureaplasmal infection of the chorioamnion is significantly associated with premature spontaneous labor and delivery. In at least some cases, it appears to be causal. Present evidence indicates that U. urealyticum is a cause of septicemia, meningitis, and pneumonia in newborn infants, particularly those born prematurely. There is strong but not definitive evidence that ureaplasmal infection of the lower respiratory tract can lead to development of chronic lung disease in very low-birth-weight infants. Although risk factors for colonization of the lower genitourinary tract have been identified, little information is available concerning risk factors for intrauterine infection and host immune responses to invasive infection. Recent establishment of animal models of respiratory and central nervous system diseases should provide an opportunity to evaluate risk factors, pathogenic mechanisms, and operative immune mechanisms. However, the most critical need is additional information concerning indications for diagnosis and treatment as well as efficacy of treatment.