Correlation functions, calculated on the ultrasonic echoes scattered by blood, provide a rich harvest of information concerning the local speed, which has often been underestimated; in particular, blood flow measurement usually yields only the longitudinal component of the velocity, even if significant information about the speed direction can be extracted. In this paper it is shown how correlation functions, calculated on the dependency of both time and space displacements, allow us to evaluate the angle Θ between ultrasound beam and blood flow; when straight vessels are considered, this single parameter, combined with the longitudinal velocity profile, permits the complete hemodynamic characterization. The underlying theory is developed and preliminary experimental results are presented