PURPOSE We present an experimental, statistical approach to estimate the size required for small vessels to become detectable with color Doppler sonography. MATERIALS AND METHODS A murine experimental tumor was examined with color Doppler sonography after injection of 1.5 ml of the contrast medium Levovist. Histologically, we measured vessel diameters inside the tumor as well as other, clearly identifiable locations. RESULTS With color Doppler at a transmit frequency of 7 MHz, vessels were only detected in the tumor's environment, but not inside. From the 95% quantiles of the vessel diameter distribution found histologically, we estimate that vessels 80-140 microns in diameter or above may be detectable with color Doppler sonography, while vessels 40 microns in diameter or smaller are indetectable. CONCLUSIONS Although a direct sonographic--histologic correlation is impossible for small vessels, a systematic assessment of the size distribution in clearly identifiable regions permits to estimate the sensitivity of color Doppler to detect blood flow in small vessels. According to our results, capillary blood flow is indetectable, and precapillary vessels may be detected only under optimal conditions.