Since 1941 much has been written about the possibility that an infection occurring in a pregnant woman may determine a malformation. The role of rubeola as a cause of embryopathies is already proved; however there has not been any convincent proof that other viral diseases may also cause them. As to the possibility of embryopathies caused by smallpox, we only found Jellife's case, in which this etiology was suggested. Regarding the malformations caused by fetopathies, toxoplasmosis and cytomegaly are the only ones well studied and characterized. Morquio, in 1903, reported three hydrocephalic patients for whose disease smallpox was suggested as the etiology; this work was not registered in the literature hence forwards. The possibility that smallpox might involve the fetus bringing about hydrocephalus was not referred to any more, although there is quite a large number of works about the consequences of this viral disease when it occurs during pregnancy. Among 459 hydrocephalic patients that were examined, we found 15 in whom it could be demonstrated that smallpox, occurring after the fourth month of pregnancy, involved the fetus determining a fetopathy which was characterized mainly by hydrocephalus. The clinical and anatomo-pathological pictures presented by these patients are in some characteristics similar to those found in cases of cytomegaly and toxoplasmosis. The study of these patients and of the literature allowed us to reach the following conclusions: 1. Smallpox occurring in a pregnant woman in the two last trimesters of pregnancy may involve the fetus determining serious encephalic damage and hydrocephalus. 2. When a child presents signs of hydrocephalus it is necessary to ask the mother whether she had an eruptive fever, smallpox in particular, during her pregnancy. Even when the mother is immune to smallpox due to vaccination or actual disease, and does not refer any rash during her pregnancy, it must be asked whether other people in the same house had the disease, for fetopathies have been attributed to a mere contact. 3. In order to know the role of the smallpox virus in the determination of malformations it is necessary to have not only a detailed story but also prospective studies from pregnant women who contracted the disease in any stage of their pregnancies or who had any contact with a diseased person.