The association between antenatal diagnostic X-ray exposure and risk of rhabdomyosarcoma in children was assessed in a national case-control study of 319 rhabdomyosarcoma cases and 319 matched controls. Data were collected by telephone interviews of subjects' parents. Overall, an odds ratio (OR) of 1.9 [95% confidence interval (CI), 1.1-3.4] was found for any X-ray examination of the mother during pregnancy. Risk was greatest for X-ray exposure during the first trimester (OR, 5.7; 95% CI, 1.2-27.8) and was also increased for the third trimester (OR, 2.0; 95% CI, 0.9-4.6), whereas second trimester exposure was not associated with increased risk. A nonsignificant increase in risk was found for any X-rays of the abdomen, pelvis, chest, or back. Increased risk was significantly associated with "other" X-ray exposures (relative risk, 2.9; 95% CI, 1.1-7.7), primarily composed of dental X-rays. The association was strongest between embryonal rhabdomyosarcoma and first trimester exposure (relative risk, 10.5; 95% CI, 1.5-458.4). This observation regarding embryonal rhabdomyosarcoma, and our previous report of an increased frequency of major malformations in rhabdomyosarcoma are compatible with findings from animal studies in which Ptc heterozygous knockout mice exhibited an increased risk of radiation-induced development defects and of spontaneously occurring embryonal rhabdomyosarcoma.