During the five-year period 1964-68 96 733 births were registered in the 28 hospitals equipped with maternity facilities in the Uppsala hospital region. Of these babies, 1 636 were born in 818 twin deliveries. Data on gestational age, sex, weight and length at birth, birth order, hospital type, congenital malformations and perinatal mortality are analysed. Altogether 17.3 per 1 000 of the children born during this period were born in multiple births. The perinatal mortality for the twin babies was 64 per 1 000 born, with the mortality higher in the less specialized hospitals than the others. Twin no. 1 suffered perinatal death in 67 cases per 1 000 and twin no. 2 in 60 cases per 1 000. For twins of primiparae the losses were 92 per 1 000 children and for twins born to multiparae 51 per 1 000. Altogether 72 per 1 000 male twins died perinatally compared to 52 per 1 000 female twins. The most heavy losses occurred among the low-weight premature twins and in these cases both twins often suffered perinatal death.