Experience on our clinical service suggested that the real-time ultrasonically measured fetal biparietal diameter (BPD) might not accurately estimate gestational age in the preterm pregnancy complicated by premature rupture of membranes (PROM). Of 2510 consecutive pregnancies for which the BPD could be obtained on ultrasound examination at or beyond 26 weeks gestation, 122 were complicated by PROM. For pregnancies with and without PROM, the relationship, if any, between BPD and gestational age at the time of examination was determined by regression analysis and the resulting equations compared by F test. The regression equations were found to be significantly different (F = 17.2, P less than 0.01); the BPD was found to underestimate the true gestational age in the presence of PROM. To model the situation in which the pregnancy is recognizably very premature, the BPD/gestational age relationship was evaluated at less than or equal to 30 weeks. In these pregnancies the correlation of gestational age with BPD was not statistically significant in the presence of PROM. These results indicate that in the presence of PROM, the BPD may not be a reliable estimator of the gestational age of the fetus. In the presence of PROM, gestational age dependent management decisions would be better based on clinical and ultrasound criteria rather than the ultrasonically determined BPD.