During the period 1965-1980, 84 patients with Wilms' tumor, neuroblastoma and Ewing's sarcoma were treated at the University of Rochester Medical Center. All patients were evaluated for the presence of congenital abnormalities. Ten of 34 (29%) patients with Wilms' tumor, 3 of 32 (9%) patients with neuroblastoma, and 0 of 18 patients with Ewing's sarcoma were so effected. In the patients with Wilms' tumor, 5 children had more than one abnormality. In this group, types of defects included genitourinary in 10 patients, central nervous system in 3, and other structural defects in 7. In the patients with neuroblastoma, the abnormalities were dissimilar. As expected, most of the 13 patients with congenital malformations were detected as having defects prior to the diagnosis of malignancy. For the patients with Wilms' tumor, median age at diagnosis for the entire group and for the subgroup with defects was 3 years, with age ranges similar. Male to female ratios were 1.3:1 and 1:1, respectively. Seventy percent of each group survived more than 5 years. For the patients with neuroblastoma, median age (range) for the entire group was 16 months (0-12 years). The patients with defects had ages of 5 days, 1 month and 9 months at diagnosis. Male to female ratio for the entire group was 0.6:1 and survival was 63% at 3 years (range 3-23 years). Those with defects are alive 3-8 years from diagnosis.