Summary. A study of 1045 twin gestations with regard to known or likely zygosity and the incidence of pre-eclampsia/eclampsia failed to reveal differences between known dizygous twins and like-sex 'presumed' and 'estimated' monozygous twins except in the 'estimated' data for multigravidae. There was a threefold increase in the incidence for twins as opposed to singleton pregnancies. These results are discussed in relation to increased conceptus-mother antigenic differences. It is suggested that the risk of gestosis in twin pregnancy involves more than a summation of that operating in two singleton pregnancies.It has been suggested that genetic incompatibility between mother and fetus may be a factor in the aetiology of pre-eclampsia (Penrose, 1946;Kalmus, 1946;Platt, Stewart, and Emery, 1958). Epidemiological evidence has been provided by Stevenson et al (1971) in a study of consanguineous marriages in the Middle East. They also recorded twin data which pointed to a higher incidence of toxaemia in unlike-sex as opposed to like-sex twin pregnancies. As all unlike-sex twins are dizygous (DZ) and a large proportion of like-sex twins are monozygous (MZ), antigenic differences between mother and conceptus would be more likely to be expressed in unlike-sex twin pregnancies. Stevenson et al considered that the higher incidence with unlike-sex twins could be attributed to this. They extrapolated their data on the basis of Weinberg's hypothesis involving the assumption that for every pair of unlike-sex DZ twins there is one like-sex DZ pair. The estimated differences in the incidence of pre-eclampsia based on this were significant at P values of <0.05 to 0.001, the syndrome being commoner in the presumed DZ cases. They regarded these findings as 'suggestive'.This report concerns a review of twin deliveries in one hospital with regard to zygosity and the incidence of late pregnancy hypertension. MethodThe records of twin gestations from 1953 to 1973 in-clusive at Leeds Maternity Hospital were studied.