The repeatability and sensitivity of Doppler ultrasonographic measurement of lateral digital arterial and venous blood flow has not been previously determined. Doppler ultrasonography was used to measure blood flow within the forelimb lateral digital vessels in one normal adult Thoroughbred horse on six occasions and in six normal adult Thoroughbred horses on three occasions, each occasion being at least 1 h apart, to determine the within- and between-horse variation. The values obtained from the right and left lateral digital vessels did not differ significantly. The within-horse coefficients of variation (CV) for arterial and venous measurements were all acceptable (<11%); the between-horse CV were acceptable (<7%) for all parameters except TaVa (average velocity of first peak of arterial waveform) and TaVb (average velocity of remainder of arterial waveform). The within-horse intraclass correlation coefficients (ICC) demonstrated excellent repeatability (≥0.71) for all parameters except venous diameter; the between-horse ICC demonstrated good to excellent repeatability (≥0.67) for all parameters except TaVb. Doppler ultrasonography can detect differences of 0.005 and 0.01 ml/min in digital arterial and venous flow, respectively, using measurements from six horses on three occasions (80% power; P<0.05). Thus, the technique is sufficiently repeatable and sensitive to be able to detect changes in flow during different physiological or pathological states or following pharmacologic intervention.