A study was made of 1655 sets of twins born between 1931 and 1975 in two clinical centres. Compared to singletons, twins were born three weeks earlier, weighed less at birth from 33 weeks to term and had smaller placentae from 21 to 22 weeks to term. The influence on birth weight fo sex was smaller in twins, the influence of parity greater. Because, compared to singletons, lower placental indices were found in twins up to 37 to 38 weeks, the conclusion is drawn that the retardation of growth in twins is to some extent due to the placenta itself. When singletons and twins of the same gestational age were compared, the mortality was found to be similar, somewhat lower in twins up to 37 to 38 weeks and higher afterwards. Monochorial twins were found to be born earlier, weigh less at birth and have a higher mortality than dichorial twins. Placental weights were not different and the conclusion is drawn that the retardation of growth in monochorial twins is to some extent due to the higher incidence of marginal and velamentous insertions of the umbilical cord associated with lower birth weights.