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In a study of the childhood leukemia experience in a California twin birth cohort, there was an apparent deficit of leukemia in twins compared to the occurrence in single births. The rough estimate of the deficit of leukemia in monozygotic twins tends to confirm that found in 2 other studies. The concordance rate for monozygotic twins was estimated to be 25%, which is in agreement with that made by MacMahon and Levy. Relative to the comparison group, the leukemic twins and their unaffected co‐twins had a significantly greater disparity in birth weights, and heavy leukemic females made the greatest contribution to this result. More first‐born twins died of leukemia than second‐born, but an independent twin birth order effect was not shown when birth weight and sex composition were controlled.

Fifty‐two childhood cancer deaths, other than from leukemia, were identified in a California twin‐birth cohort. The occurrence of “other cancer” was similar to leukemia (previously reported) in that there was an overall deficit of twin deaths characterized by a deficit of females, second‐born, and twins weighing less than 51/2 pounds at birth. Contrasts in the cancer findings were that monozygotic twins did not appear to be underrepresented among “other cancer” deaths as they did for leukemia, and an independent effect of intra‐pair differences in birth weight, which had been found for leukemia, could not be detected for other forms of cancer. Three twin sets were found in this twin population with concordant cancer; 2 with leukemia and one with gonadoblastoma.