Brush cytology in the diagnosis of colonic neoplasms. During a three-year period (1986-1988), 234 colonic brush specimens were received in the authors' laboratory. Nine samples (4%) were deemed unsatisfactory for evaluation because of inadequate cellularity and/or poor fixation. In 11 cases concomitant or follow-up histologic specimens were not available. The remaining 214 specimens included 82 malignant neoplasms, 88 neoplastic polyps (adenomas), and 44 nonneoplastic lesions. Sixty-seven (82%) of malignant neoplasms were correctly diagnosed by brush cytology. Three cases of adenoma with severe dysplasia or in situ carcinoma were diagnosed as adenocarcinoma by cytology. No false-positive diagnoses were made of nonneoplastic lesions. Brush cytology was found to be a more sensitive technique in the diagnosis of colon cancer than endoscopic biopsy (82% and 74% sensitivity, respectively). The combination of the two techniques increased the sensitivity to 90% and improved the overall accuracy of the test. Seventy-one (82%) of the colonic adenomas were correctly diagnosed by cytology. Brush cytology is a convenient, safe, and accurate technique which should be used concurrently with endoscopic biopsy or polypectomy. 