The purpose of this study was to determine the prevalence and clinical significance of microbial invasion of the amniotic cavity during spontaneous parturition at term. Amniotic fluid was retrieved by transabdominal amniocentesis from 90 women in spontaneous term labor with intact membranes. Fluid was cultured for aerobic and anaerobic bacteria, including Mycoplasma. The prevalence of positive amniotic fluid cultures was 18.8% (17/90). The most common microbial isolates were Ureaplasma urealyticum, Streptococcus agalactiae, Lactobacillus species and Mycoplasma hominis. Clinical signs of chorioamnionitis were present in three patients, and only one of them had a positive amniotic fluid culture. Five patients (5.5%) had puerperal endometritis; three of them had an amniotic fluid culture positive for microorganisms. All neonates were free of clinical signs of infection. The data indicate that microbial invasion of the amniotic cavity occurs frequently during spontaneous labor at term, and it may be both the cause and the consequence of labor.