Colonic blood flow was measured with laser Doppler flowmetry during operation in 62 patients and during coloscopy in 15 patients. In 18 subjects mucosal and serosal laser Doppler signals were compared during 'resting' conditions, vascular occlusion, and reactive hyperemia. The mucosal (n = 36) and serosal (n = 36) flowmeter signals were of the same order of magnitude throughout the whole range of flowmeter signals (r = 0.96; p less than 0.001). In eight subjects total venous outflow from a colonic segment was measured simultaneously with flowmeter recordings from the serosal side. A correlation coefficient of 0.95 (n = 46; p less than 0.001) was obtained between total intestinal blood flow and serosal flowmeter signal during 'resting' and reduced blood flows. In this flow range a calibration curve was constructed for interpretation of the flowmeter signals in absolute flow units. The wall thickness of the bowel determined the quantitative relationship between flowmeter signal and total intestinal blood flow. It is concluded that laser Doppler flowmetry represents a potentially very interesting non-invasive, continuous method for the quantitative study of human intestinal blood flow.