OBJECTIVE: To validate a transit time ultrasound flowmeter (CardioMed CM 4000) for measuring blood flow in isolated colonic mesenteric arteries. DESIGN: Experimental and clinical study. SETTING: Teaching hospital, Denmark. ANIMALS AND SUBJECTS: One female pig, and 6 patients being operated on for carcinoma of the sigmoid colon and rectum. INTERVENTIONS: Volume blood flow measured by Cardiomed CM 4000 and pump withdrawal flow recordings. MAIN OUTCOME MEASURES: Correlation between the two methods. RESULTS: There was good agreement between transit time flow recordings and pump withdrawal flow recordings (correlation coefficient of 1.0). Of the differences between the two methods, 95% were between -0.16 ml min(-1) and 1.29 ml min(-1), mean 0.57 ml min(-1), or (in percentages) 105, 95% lying between 97-115. There was also good reproducibility in transit time flow recordings, the mean difference between repeated measurements being 0.06 ml min(-1), 95% lying between -0.66 ml min(-1) and 0.78 ml min(-1). CONCLUSION: Ultrasound transit time flow recordings gave precise measurements of blood flow in isolated colonic mesenteric arteries.