Ray-based simulations have been shown to generate impressively realistic ultrasound images in interactive frame rates. Recent efforts used GPU-based surface ray-tracing to simulate complex ultrasound interactions such as multiple reflections and refractions. These methods are restricted to perfectly specular reflections (i.e., following only a single reflective/refractive ray), whereas real tissue exhibits roughness of varying degree at tissue interfaces, causing partly diffuse reflections and refractions. Such surface interactions are significantly more complex and can in general not be handled by such deterministic ray-tracing approaches. However, they can be efficiently computed by Monte-Carlo sampling techniques, where many ray paths are generated with respect to a probability distribution. In this paper we introduce Monte-Carlo ray-tracing for ultrasound. This enables the realistic simulation of ultrasound interactions such as soft shadows and fuzzy reflections. We discuss how to properly weight the contribution of each ray path in order to simulate the behavior of a beamformed ultrasound signal. Tracing many individual rays per transducer element is easily parallelizable on modern GPUs, as opposed to previous approaches based on recursive binary ray-tracing.