This study assessed within- and between-day variability of human superior mesenteric artery blood flow (SMABF) using transcutaneous Doppler ultrasound. Two groups of healthy volunteers were studied after fasting overnight; six subjects were studied for one visit of 90 min duration (group 1) and six other subjects attended the laboratory for three separate visits of 30 min duration (group 2). Measurements of SMABF were made by a single, trained observer every 15 min, the angle of insonation of the Doppler beam being set both < 30 degrees and > 30 degrees each time. Within a day (group 1), the angle of insonation had little effect upon SMABF measurements (values differed by less than 6%) and variability was low (mean coefficient of variation below 5%). SMABF measurements between days (group 2) were consistent (mean coefficient of variation ranging from 6 to 12%), but measurements made at angles of < 30 degrees had slightly lower standard error of the mean (SEM) and coefficient of variation (CV) than those above 30 degrees. Thus, in fasted subjects there is small within-visit or between-visit variability in SMABF assessed by a single trained observer using Doppler ultrasound. Slightly less variability is observed when scanning at angles of < 30 degrees, but this effect was not substantial and the correction factor for angle contained within the software of the ultrasound machine appeared to be reliable.