It is very important to appraise cephalo-pelvic disproportion (CPD), and to predict the necessity of extraction of the after-coming head on full-term breech presentation. Subjects were 47 cases of full-term breech presentation which delivered at our hospital during the 5-year period from 1979 to 1983. They were divided into the following groups: group A Assisted breech delivery. group B Extraction of after-coming head. group C Abdominal delivery for secondary inertia. In each case we subtracted the fetal occipitofrontal diameter from the individual pelvic diameter and compared the results for groups A and B. There were significant differences between groups A and B in the mean values (cm) for the following: True antero-posterior diameter of the wide; 1.341 +/- 1.009 and 0.380 +/- 0.379 (p less than 0.001). Antero-posterior diameter of the Fukushima's wide; 2.005 +/- 0.906 and 0.860 +/- 0.502 (p less than 0.01). Posterior-sagittal diameter of the wide; -5.300 +/- 1.026 and -6.020 +/- 0.340 (p less than 0.05). Antero-posterior diameter of the midpelvis; 1.414 +/- 1.035 and -0.110 +/- 0.718 Posterior-sagittal diameter of the midpelvis (Caldwell-Moloy); -5.759 +/- 1.041 and -6.560 +/- 0.573 (p less than 0.05). Posteriol-sagittal diameter of the midpelvis (Thoms): -6.173 +/- 1.083 and -7.430 +/- 0.750 (p less than 0.01). Furthermore, there were also significant differences between groups A and C: The margins of safety values for each pelvic diameter (cm) were as follows: Antero-posterior diameter of the midpelvis; (13.1): Posterior-sagittal diameter of the midpelvis (Thoms); (5.5): Inter-spinous diameter; (11.0): Inter-tuberous diameter; (11.3).(ABSTRACT TRUNCATED AT 250 WORDS)