The use of brush cytology as an aid in the preoperative diagnosis of rectal cancer was prospectively assessed at 328 examinations in 289 consecutive patients with rectal lesions suspicious of carcinoma. Forty-five patients were reported as having benign polyps. There was 97 per cent agreement between conventional biopsy and cytology brushings for this group. Some 249 patients underwent a subsequent resection allowing comparison with formal histology. Forceps biopsy produced a true-positive diagnosis of cancer in 218 patients (89·7 per cent) and cytology in 222 (91 per cent). The sensitivity for forceps biopsy and cytology was 81 and 83 per cent respectively, with a combined sensitivity of 98 per cent. It is concluded that the addition of cytology to forceps biopsy increases the diagnostic yield in a single examination, and may be a complementary method of establishing the diagnosis of rectal lesions.