Any disease occurring in one identical twin may be expected to occur in the other at the same time or at some future time. In a recent report (1953) of sympathicoblastomas occurring in 3-month-old identical twins Lee1 stated that "when a tumor, especially a malignant tumor, is recognized in one of identical twins, there is a considerable possibility that a malignant lesion, usually, but not always, of a similar type will appear in the other twin."
In the study of disease in twins one of the difficult problems is the identification of identical (monozygotic) twins. Unidentical (dizygotic) twins of the same sex may so closely resemble each other that they may not be distinguished from identical twins unless one placenta for both is recorded at birth. Rosanoff and associates2 emphasize this problem, as shown in Table 1, taken from their report of a study of schizophrenia in 142