The presumable relationship between viruses and malignant disease has been studied. Approximately 4000 assays for antibodies to adenovirus were carried out with sera of patients with malignant and nonmalignant diseases of the urogenital organs and other organic diseases. The search was directed in the first place at antibodies to the early non-virion antigens of adenovirus-12 of oncogenic properties. On the evidence of the complement fixation reaction antibodies to adenoviruses were found to be less frequent in malignant tumors and in hypertrophy of the prostate than in the control group. Antibody to the non-virion antigen of adenoviruses was found in 53 per cent of patients with neoplastic diseases and prostatic hypertrophy, in 18 per cent of patients with urogenital diseases other than tumors and in 4 per cent of those with other organic diseases. The results point to a possible role of adenoviruses in tumors of the urogenital organs.