In order to study the complications and therapeutic outcome of twin pregnancies, a retrospective survey was carried out in the University Central Hospital of Oulu. Twin deliveries during 1965-1973 numbered altogether 335, and their relative frequency was 1.7 %. The deliveries took place in the 37.2th gestational week on an average. The mean weights of the infants were 2590 g (A) and 2562 g (B). Perinatal mortality in the total series was 9.3 % (A 9.0 % and B 9.6 %). Pregnancy terminated before the 37th week in 29.2% of the cases. Perinatal mortality in this group was 27.0 %, while the corresponding value in the full-term group was 1.7 %. The perinatal mortality of primigravidas (14.1 %) was about twofold compared with that of the multiparas (7.2 %). Twin pregnancies were complicated by hyperemesis gravidarum, pre-eclampsia, anaemia, pyelonephritis and hepatosis more often than were the single pregnancies. The complications which contributed towards an increase of perinatal mortality included uterine bleeding in early and late pregnancy, hydramnion and superimposed pre-eclampsia. The complications generally associated with twin pregnancies and the increased perinatal mortality involved require that mothers with twin pregnancy, particularly primigravidas, should be subjected to intensified follow-up and treatment.