Twins show significant differences in size and gestational age at delivery when compared with singletons; their intrauterine environment differs endocrinologically and immunologically. We have therefore investigated measures of health longitudinally in a population-based cohort in the west of England comprising 9866 singletons and 222 twins. Zygosity of twins was determined by placental histology, and DNA fingerprinting when this was equivocal. Data collected by questionnaire and regular medical follow up were analysed up to 18 months of age in order to compare twins with singletons. There were no significant differences in terms of death rates, or viral upper respiratory tract infections or common exanthema when twins and singletons were examined. However in the first six months of life there were more episodes of diarrhoea and napkin dermatitis among twins (p<0.001), but less vomiting, particularly in dizygotic twins (p<0.001). There was less colic reported in monozygotic twins up to 18 months of age (p<0.001). Differences were also identified regarding atopic dermatitis, which was less frequent in dizygotic twins than either singletons or monozygotic twins (p<0.05). However there were no significant differences in wheezing identified by this age. These figures suggest that factors in utero, or in an environment with two infants rather than one significantly alter several aspects of child health. There are indications that gut function and allergic disease may differ in twins: this pattern requires investigation at a later age in order to check its significance.