Objective: Our purpose was to determine the utility of cervicovaginal washing prolactin assay in prediction of preterm birth in women without rupture of membranes. Methods: Sixty-six women with normal singleton pregnancy were submitted to cericovaginal washing and serum prolactin assays. The latency period to delivery and gestational age at admission and at delivery were also recorded. According to uterine contractions and obstetrical history regarding the previous preterm delivery, the pregnant women were divided into 4 groups: 18 symptomatic (group 1) and 15 asymptomatic (group 2) pregnancies who had previously had preterm delivery, and 18 symptomatic (group 3) and 15 asymptomatic (group 4) pregnancies without a history of prior preterm delivery were enrolled in the study. Results: The cervicovaginal washing prolactin concentrations were significantly higher in groups 1 and 3 than in group 4 (P < 0.0083). With respect to the latency period to delivery and the birth weeks, groups 2 and 4 were significantly higher than groups 1 and 3 (F < 0.0001). In the evaluation of the whole group, a significant negative correlation was observed both between cervicovaginal washing prolactin concentrations and the lapsed times to delivery, and the gestational ages at delivery. The finding of a cervicovaginal washing prolactin value exceeding 50 ng/ml in the 12 days preceding preterm delivery had sensitivity, specificity, positive and negative predictive values of 65%, 95%, 86%, and 81%, respectively. Conclusions: A cervicovaginal washing prolactin value more than 50 ng/ml precedes preterm delivery within 12 days at > 29 weeks. The easy application, the good feasibility, the success in identifying pregnancies at risk for preterm labor, and the cost effectiveness suggests cervicovaginal washing prolactin assay as a biochemical marker for preterm delivery. (C) 1997 International Federation of Gynecology and Obstetrics.