ARECENT SURVEY showed that the incidence of neoplasia in a group of children who had been treated with x-rays in infancy for enlargement of the thymus gland was higher than that in either their untreated siblings or in children of the same age in the population at large (1, 2). In particular, the number of cases of leukemia and thyroid cancer in the irradiated children far exceeded the number expected in a normal group of comparable size and age distribution. Although the findings are consistent with reports of an increased incidence of malignant disease, especially leukemia, in other human populations exposed to ionizing radiation (3-7), certain features of this study are sufficiently unusual to deserve further investigation. First, the doses of radiation were smaller than those formerly believed to be carcinogenic. Second, whereas the thyroid neoplasia and other cancers were found only in children exposed to comparatively high doses of x-rays (more than 200 r), leukemia sometimes occurred in children receiving less than 200 r. Third, because almost all children were selected for treatment on the basis of a diagnosis of thymic enlargement, it is impossible to differentiate the role of this factor from that of irradiation in the development of leukemia and other malignant diseases. The purpose of the present study is to obtain further information concerning the incidence of malignant disease in children treated with x-rays. The irradiated children in the series to be reported are of particular interest in that most of them had thymus glands of normal size. In this 3 per cent of the entire group. Although these children were usually asymptomatic, 80 per cent were treated with 200 to 450 r to the anterior mediastinum. In the course of the study, it was noted that signs and symptoms attributed to thymic enlargement never occurred in treated children who had enlarged thymus glands at birth. Since upper respiratory infections also seemed to be less prevalent in the treated children, it was decided to give all children born between June 1944 and May 1946 small doses of x-rays to the manubrial region. Ninety-five per cent of the 1,340 children so treated were given 150 r and the remainder received 75 to 100 r (Table I). A survey of these children, conducted by mail in 1948, reaching about half the total number, showed no apparent increase in either morbidity or mortality. The radiation factors used in treatment were: 140 kvp; filtration, 3 mm. Al; skin-target distance, 8 to 9 in.; port size, 4X4 cm.; port position, over the manubrium. Increments of 75-150 r were usually given at one- to two-day intervals. All children were treated within seven days after birth; children with illnesses not usually ascribed to thymic enlargement were excluded from the series. The method of interviewing parents of the treated children was the same as that used by Simpson and Hempelmann (2) except that the telephone was used wherever possible.