In order to investigate the associations between sources of exposure to ionizing radiation and childhood cancer in Germany, a matched case-control study including children under the age of 15 years was conducted. Cases were identified from the German Childhood Cancer Registry; controls came from population registration offices. Exposure was assessed via questionnaires and parental interviews. The study comprises 1184 leukemia cases, 234 non-Hodgkin's lymphomas, 940 solid tumors, and 2588 controls. Preconception parental occupational exposures were positively but not statistically significantly related to all of the cancer types in the study. Maternal occupational exposure during pregnancy was a risk factor for childhood lymphomas [odds ratio (OR) = 3.87, 95% confidence interval (CI): 1.54-9.75] but not for leukemia or solid tumors. ORs for parental occupational exposures were noticeably more pronounced in leukemia cases who were diagnosed in their first 18 months of life. A preconception paternal occupation in the nuclear industry under dosimetric surveillance yielded an OR of 1.80 (95% CI: 0.71-4.58). However, radiation doses of these fathers were often unknown or below the level of detection, and no dose exceeded 30 mSv. Prenatal X-ray examinations of the father (but not of the mother) were significantly related to childhood leukemia (OR = 1.33; 95% CI: 1.10-1.61). No effects were observed for postnatal X-ray examinations of the child. The results suggest that, in Germany at present, exposures to ionizing radiation do not play a noticeable role in the development of childhood cancers. The major strengths of the study are its size and the population basis. The validity of the data from parental questionnaires and the possibility of residual confounding by socioeconomic factors are potential drawbacks.