OBJECTIVE: To evaluate the potential role of amniotic fluid (AF) interleukin (IL)-6 as a predictor of preterm delivery and to consider possible explanations for the proportion of women with elevated AF IL-6 who deliver preterm yet lack microbiologically detectable intra-amniotic infection. DATA SOURCES: We searched the English language human literature in MEDLINE, 1966 through September 1999, using the keywords "labor/infant," "premature," "cytokines/interleukin-6," and "AF." We also examined abstracts from the 1999 meetings of the Society for Maternal-Fetal Medicine and the Society for Epidemiologic Research. We identified other studies by reviewing the reference lists of published articles. METHODS OF STUDY SELECTION: The MEDLINE search yielded 55 citations. We focused on studies that reported on the association between AF IL-6 and preterm delivery. TABULATION, INTEGRATION, AND RESULTS: There is consensus in the literature that elevated AF IL-6 is a stronger predictor of preterm delivery than intra-amniotic infection detected by either microbiologic culture or polymerase chain reaction (PCR). Among women with elevated AF IL-6, PCR could detect a higher proportion of intra-amniotic infection than culture. A number of women with elevated AF IL-6 (33-70%) deliver preterm and do not have evidence of intra-amniotic infection by either culture or PCR. Possible explanations for this observation are considered. CONCLUSION: Elevated AF IL-6 is strongly associated with preterm delivery and merits future consideration in clinical settings to predict preterm delivery and guide patient care. Development of improved polymerase chain reaction-based clinical methods to detect intra-amniotic infection is necessary to better understand the relationship between elevated AF IL-6, intra-amniotic infection, and preterm delivery.