A production six-hole conical sac-type nozzle incorporating a quartz window in one of the injection holes has been used in order to visualize the flow under cavitating flow conditions. Simultaneous variation of both the injection and the back chamber pressures allowed images to be obtained at various cavitation and Reynolds numbers for two different fixed needle lifts corresponding to the first- and the second-stage lift of two-stage injectors. The flow visualization system was based on a fast and high resolution CCD camera equipped with high magnification lenses which allowed details of the various flow regimes formed inside the injection hole to be identified. From the obtained images both hole cavitation initiated at the top inlet corner of the hole as well as string cavitation formed inside the sac volume and entering into the hole from the bottom corner, were identified to occur at different cavitation and Reynolds numbers. Comparison of the cavitation images obtained in the real size nozzle with those obtained in an enlarged fully transparent acrylic nozzle replica has confirmed that similar flow regimes may form inside the injection hole of real size and large scale injectors. Despite the similarity in the macroscopic cavitation structures that were found to depend on cavitation intensity, the transient development of cavitation bubbles was identified to be different inside the two nozzles due to the different residence and life time of the moving bubbles. Based on the flow similarity between the real size and enlarged multi-hole nozzles and previous extensive experience on the cavitating flow characteristics in large scale nozzles, better understanding of the cavitating flow in real size nozzles operating under Diesel engine conditions has been obtained, which can guide the development of relevant computer models.