Diagnostic X-ray examinations as a potential risk for neoplasia were investigated in a prospective study of 55,908 women who participated in the Collaborative Perinatal Project of the National Institute of Neurological and Communicative Disorders and Stroke. The X-ray exposure histories of 145 mothers whose children developed neoplasms and 290 matched controls were examined. Of the childhood neoplasms, 40 were malignant and 105 were benign. Generally, the data were consistent with increased risk of malignant neoplasms among children of women exposed to X-rays before and during pregnancy, with a somewhat higher relative risk estimate for preconception exposure. No significant association of X-ray exposure and benign neoplasms was detected.