A TSI laser Doppler flowmeter was used via a rigid sigmoidoscope in 20 consecutive patients with rectal carcinoma. Blood flow and velocity was evaluated at the tumour centre, tumour edge and adjacent normal mucosa; blood flow in the centre of the tumour was significantly higher than in normal mucosa (P = 0.05) and the tumour edge (P = 0.037). Two patients with ulcerated lesions had much lower blood flow in the centre of the ulcerative lesion.