Three hundred sixty cytologic specimens obtained by colonoscopic brushing from 336 patients were compared with biopsy specimens simultaneously obtained for histologic examination. Of the cytologic specimens, 160 (44%) were positive for malignant cells, 37 (10%) contained suspicious cells, 54 (15%) had atypical glandular cells, 107 were cytologically negative, and 2 were considered unsatisfactory. Eight-four percent of the patients with cytologically positive smears and 54% of those with suspicious smears had malignant neoplasms in the simultaneously obtained tissue biopsies. Of the patients with follow-up, all with cytologically positive findings and nine with suspicious findings on the initial cytologic examination and simultaneous negative tissue biopsies, were subsequently found to have carcinoma of the colon. Cytology proved to have a sensitivity of 0.73 and a specificity of 1.00 while tissue biopsy showed a sensitivity of 0.81 and a specificity of 1.00. By combining the two methods, the sensitivity increased to 0.92. It is concluded that cytologic examination of colonic brushings is a highly accurate and reliable technique for the detection of malignant neoplasms of the colon and can preempt the use of biopsy forceps.