An ultrasound system performs duplex colorflow and spectral Doppler imaging, with the spectral Doppler interrogation performed at a sample volume location shown on the colorflow image. The colorflow image is displayed in a color box overlaid on a co-registered B mode image. A color box position and steering angle processor analyzes the spatial Doppler data and automatically sets the color box angle and location over a blood vessel for optimal Doppler sensitivity and accuracy. The processor may also automatically set the flow angle correction cursor in alignment with the direction of flow. In a preferred embodiment these optimization adjustments are made automatically and continuously as a user pauses at points for Doppler measurements along a length of the blood vessel.