Patients attending a twin pregnancy antenatal clinic underwent cervical palpation at each visit to calculate a cervical score by subtracting dilatation from length. Prediction of the onset of labor by cervical assessment was studied in 223 already parous women with twin pregnancies who ultimately labored spontaneously. The lower the score, the shorter was the mean time until delivery, although there existed a subgroup of patients who had ripe cervices for several weeks before the onset of labor. The cervical score itself appeared to give better prediction of labor than did changes in the score. Cervical assessment identified a group of twin pregnancies at especially high risk of preterm labor, in that 76% of patients with a score of -2 or less at or before 34 weeks delivered preterm. There was no evidence that labor was precipitated by vaginal examination, although this requires further study.