This retrospective clinical study was done based under the delivery records of 146 cases of twin pregnancies in Yonsei University, Severance Hospital and Wonju Christian Hospital from 1977 through 1980 with particular interest in the second twins. Clinical factors, Apgar scores related to neonatal prognosis, and perinatal mortality rate were statistically analyzed and the following results were obtained. The mean - birthweight for A (first) twins was 2,377 grams and for B (second) twins 2,296 grams. In comparison of the percentage of low-birth-weight (less than 2,500 grams) infants, there were more small B twins, but the difference was statistically not significant. Because there were no significant statistical differences in birth weight-grouping between A and B twins, they could be compared with the comprehensive Apgar scores, but this method was also shown to be statistically not significant. In comparison of the percentage of breech deliveries in A and B twins, the percentage was more than three times in B twin (A twin, 7.5 per cent; B twin, 24.0 per cent), and the difference was statistically very significant (p < 0.0005). There were no significant statistical differences between the Apgar scores of A and B twins in reference to the manner of delivery. According to the manner of delivery of B twins, spontaneous vertex delivery and total breech extraction revealed higher infant mortality rate than others. B twins presented by the vertex in 88 cases (61.0 per cent), by the breech in 55 cases (37.0 per cent), and by the shoulder in 3 cases (2.0 per cent). And this therefore disclosed no significant statistical differences in Apgar scores in relation to the presentation. The duration of labor appeared to have no clear correlation with the Apgar scores and the perinatal mortality of A and B twins. The delivery interval between A and B twins was 11.9 minutes on an. averge, varying from 3 to 65 minutes. The length of this interval was not found to have any significant effect on the Apgar scores and the perinatal mortality rate of B twin.