Tumours of the central nervous system comprise 23% of all childhood cancers and form the most common group of solid malignancies. Little is know about their aetiology. The present report concerns the results of a case-control study of 78 incident cases of central nervous system tumours in children. No case-control differences were detected for the following: pre-natal diagnostic X-rays, general anaesthetics during pregnancy, pregnancy infections, pregnancy drugs (including sedatives, tranquillizers and anti-convulsants), alcohol consumption in pregnancy, child's birthweight, breast-feeding, childhood illnesses, previous medication in the child. A significant excess of case mothers had suffered from diseases of the nervous system (RR 2.6). There was a deficit of children who had been immunised among the case children which approached significance, and an excess of congenital abnormalities among cases which also approached significance. There was a small excess of neoplastic disease among case parents. The results of this study suggest that in our patients genetic rather than environmental factors are more important, but the small numbers included in the present study meant that no definite conclusions could be reached.