The cause of adenocarcinoma of the colon has not been proven and a viral association has not been reported with this disease. A sensitive biochemical technique for the detection of viral nucleic acid was used to determine whether one type of herpesvirus, namely a human strain of cytomegalovirus, was uniquely present in tumour tissue. 24 specimens of colon, obtained at surgery from 14 patients, were analysed by membrane CR.N.A.-D.N.A. hybridisation. 4 of 7 tumours of the colon were definitely positive for C.M.V. D.N.A. (≥2 genome-equivalents/cell), or repeatedly showed more than 1 genome-equivalent/cell. Macroscopically normal colon 5 cm from the tumour, as well as histologically normal and abnormal colon from a control population with Crohn's disease, were uniformly negative for C.M.V. D.N.A., except for 1 specimen which was macroscopically normal tissue from a patient with carcinoma of the colon. Among patients with condiitons that predispose to adenocarcinoma of the colon, 1 of 2 patients with familial polyposis and 1 of 3 with ulcerative colitis harboured in their colon ≥2 genome-equivalents of C.M.V. D.N.A. per cell, and another ulcerative colitis patient had 1-2 genome-equivalents/cell.