The purpose of this study was to examine the relationship between intraamniotic infection and the onset of labor in patients with preterm premature rupture of the membranes. Two hundred and thirty consecutive patients were admitted with premature rupture of the membranes to Yale-New Haven Hospital from January 1985 to July 1987. Amniotic fluid was retrieved by amniocentesis from 96% (221/230). Sixty-one patients were in labor on admission (27.6%, 61/221) and 39% of them (24/61) had a positive amniotic fluid culture. Patients in labor on admission were more likely to have a positive amniotic fluid culture than those who were not in labor on admission (24/61 versus 41/160, p = 0.049). Of the 160-patients who were not in labor on admission, 81 subsequently went into spontaneous labor; microbiologic information at the time of labor was known in 48 of these patients (59.2%). Seventy-five percent (36/48) of these patients had a positive amniotic fluid culture. The incidence of intraamniotic infection in quiescent women who subsequently went into labor was higher than that of patients admitted in active labor (75% versus 39%, p = 0.0004). These results provide a basis for the clinical impression that the onset of labor in women with preterm premature rupture of the membranes is associated with a subclinical intraamniotic infection. The mechanisms responsible for the onset of labor in women without an intraamniotic infection may be associated with an extraamniotic infection (e.g., deciduitis) or a noninfectious process.