We investigated whether human adenovirus type 4 (Ad4) might cause cancer in humans. Cell culture and animal model studies indicated that Ad4-induced human tumors should contain Ad4 DNA sequences. Thus Ad4 DNA was labeled in vitro (sp act, approximately 10(7) 3H counts/min/microgram) and hybridized in liquid phase with DNA extracted from human tumors. No viral sequences were detected in 31 squamous cell carcinomas of the lung, 5 adenocarcinomas of the lung, 4 oat cell carcinomas of the lung, 4 carcinomas of the stomach, 10 carcinomas of the colon, 3 carcinomas of the kidney, 3 carcinomas of the breast, 2 carcinomas of the ovary, and 6 Hodgkin's lymphomas. Reconstruction experiments with added Ad4 DNA indicated that the probe detected about 1 copy of 5% of the Ad4 genome per tumor cell. Therefore, these data were strong evidence (but did not prove) that none of these particular tumors were induced by Ad4.