About 22 mio m³ of rock fell from a cliff near the village of Randa (10 km north of Zermatt, Switzerland) on 18 April 1991. A second retrogressive rockslide of about 7 miom³ followed on 9 May 1991. At present, a rock mass situated above the scarp is still slowly moving toward the valley, involving several miom³ of rock. A kinematic approach to study of this well-documented rockslide was made “a posteriori” in order to identify the parameters relevant to the detection of such failures involving large volumes of rock. A 3-D model of the pre-rockslide geometry is presented, and is used to interpret the geostructural, hydrogeological, and chronological data. The steepness of the cliff, the massive lithology (mainly orthogneiss), the location on a topographic ridge outcropping at the confluence between a glacial cirque and the main valley, and the existence of previous events of instability were the pre-existing field conditions that affected the stability of the area. The structural cause of instability was a 30 dipping, more than 500-m-long, persistent fault, which cut the base of the rock face. Together with a steeply dipping set of  persistent joints, this basal discontinuity delimited a 20- mio-m³ rock block, with a potential sliding direction approximately parallel to the axis of the valley. To the North, the fractures delimiting the unstable mass were less persistent and separated by rock bridges; this rock volume acted as key block.