Rhabdomyosarcoma (RMS) is an uncommon malignant soft tissue sarcoma whose cause is largely unknown. Reported risk factors include genetic alterations (e.g., p53 mutations, a defective gene at 11p15.5, or specific chromosomal translocation of t(2:13)), and parents' use of drugs around the time of conception. We present results from a national, case-control study of 249 RMS cases (170 males and 79 females) and 302 controls (196 males and 106 females). The cases, aged 0-20 years at diagnosis, were identified via the Intergroup RMS Study-III during 1982-1988. Controls were selected by random digit telephone dialing. As a supplement to the original study, information on genetic diseases and birth defects (BD) was collected from the subjects' parents by telephone interview. Fifty-six (22.5%) cases and 55 (18.2%) controls were reported to have genetic diseases or BD (odds ratio [OR] = 1.30,95% confidence interval [CI] = 0.85-2.02, P = .21). The case group had a significantly higher frequency of neurofibromatosis type I (NF1) than did the control group, i.e., five cases (2.0%) had NF1 vs. zero controls (P = .02). The case group also had a higher frequency of major BDs than did the control group (6.0% vs. 2.6%, OR = 2.36, 95% CI = 0.92-6.52, P = .05). However, this excess was only observed in males (7.6% vs. 2.6%, OR = 3.16, 95% CI = 1.02-10.41, P = .02). Among the 15 cases having both RMS and major BDs, six (40.0%) had both conditions in the same regional anatomic site: Two (13.3%) had both in the extremities, two (13.3%) in the genitourinary system, and two in the head and neck. These findings suggest that common genetic mechanisms or in utero exposures may be involved in the development of many childhood tumors and congenital abnormalities.