Abstract Cavitation developing upstream and inside the micro-channel orifices of transparent multi-hole fuel injector nozzles has been characterised using a high speed visualisation system. Images have been obtained with short exposure time and sufficiently high spatial and temporal resolution, freezing the formation of cavitation bubbles and their further development during every single injection cycle. Post processing of statistically large number of images collected from many successive injection events and numerous identical nozzles has allowed for the first time estimation of the ensemble average cavitation image and its standard deviation. The instantaneous images reveal the formation of a variety of complex and interacting two-phase flow regimes. Vapour bubbles have been found to exist inside the nozzle prior to start of injection. These bubbles originate from the previous injection cycle as they have not been evacuated from the nozzle and hence, they remain trapped, altering the flow of the subsequent injection event. During the opening and closing stages of the needle valve that controls the fuel flow through the nozzle, cavitation is found to form in the valve’s seat area. Subsequently, vortex or ‘string’ cavitation has been recorded to take place in a rather chaotic manner; its life time and most probable location of appearance have been estimated. The ensemble average images reveal the probability of cavitation appearance at a specific location within the nozzle and the micro-channel flow orifice. The standard deviation from the mean reveals locations with significant cycle-to-cycle variations of the flow. These are linked to significant deviations from the mean of the fuel spray dispersion angle forming downstream of the nozzle exit.