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1. The neo-natal death rate among premature babies is very much higher than that among mature babies, and as a result of the changing pattern in infant mortality in recent years prematurity has become relatively more important as a cause, or contributing cause, of infant deaths. 2. When surviving premature children are compared with surviving mature children it has been found that the premature children are at a disadvantage in various respects. 3. For these reasons it is important to attempt to reduce the incidence of prematurity, but this object is hampered by inadequate knowledge of the aetiology of prematurity. 4. A premature child is defined as a child who weighs 5 1/2 lbs. (2500 G.) or less at birth, regardless of the length of gestation. In spite of its faults, this is the best definition available. 5. The aetiology of prematurity is not fully understood. In about half the cases of prematurity which occur no cause can be defined. In the remaining half the prematurity is associated with complications of pregnancy and it is generally assumed that these complications are the cause of the prematurity, but the way by which this is brought about is not known. It is known that a large number of factors such as social class, maternal age, and parity influence the prematurity rate. It is also known that prematurity tends to be repeated and this tendency may be associated with a tendency to abort and show signs of toxaemia. 6. The aim of this study was to examine the occurrence of prematurity against the background of the reproductive tendencies of the mothers of premature children. In order to do this, we interviewed the mothers of 515 single-born premature children who had been born in Glasgow during the year 1950, and obtained the obstetric histories of these mothers. There does not appear to be any bias in the sample which would influence the conclusions which will be drawn. 7. The incidence of prematurity among the single-born sib- lings of these premature children was 26.4%. It was shown that in a number of cases the prematurity in 1950 might have been caused by factors associated with that particular pregnancy, but if these cases were excluded it would be reasonable to postulate that the prematurity in the remaining cases was due to a predisposition on the part of the mothers to have premature babies. 8. This predisposition to prematurity is made up of two tendencies: one to have mature babies of low birth weight which are by definition premature, and the other to have abnormally short pregnancies. 9. There is no fundamental difference between the reproductive histories of mothers who have premature babies after uncomplicated pregnancies and mothers who have premature babies after complicated pregnancies. Such differences as appear to exist are due to a tendency for the mothers who had complicated pregnancies to have still-births and abortions instead of premature live-births among their other pregnancies, and to the fact that most of the mature children of low birth weight who are classified as premature are born after uncomplicated pregnancies. There is an important relationship between prematurity and complications of pregnancy in that prematurity occurs more often in the presence of complications than in their absence, but the similarity in reproductive histories, and the absence of satisfactory evidence to the contrary, justifies the rejection of the view that prematurity is caused by complications of pregnancy. 10. The incidence of abortions among the other pregnancies of the mothers of these premature children was unduly high, especially when the birth weight of the premature child had been low. This and other evidence led to the conclusion that there is some common factor in the aetiologies of abortion, prematurity, and toxaemia. 11. The main conclusion drawn from this study is that most cases of prematurity are due to a maternal predisposition to have premature babies. This may simply be a tendency for certain mothers, probably of small stature, to have full-time babies of low birth weight,but more often it is due to a tendency to have abnormally short pregnancies. These short pregnancies are caused by a maternal-foetal reaction due ultimately to the genetic constitution of the foetus. This conclusion is supported by the finding that there is a relationship between a history of prematurity in the family of either parent and the incidence of prematurity among their children. Most of the remaining cases of prematurity, which are not due to a maternal predisposition, are caused by the action of factors associated with first pregnancies, high parity, and pregnancies during the later child-bearing years. A small residue is due to various other causes.