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The paper analyses the landslide dam phenomenon in an area of the central-northern Apennines (Italy). A census of 53 landslides was carried out, identifying the salient parameters of both the landslides and the riverbed–alluvial plain systems. Correlations between these parameters were tested, in order to characterize the phenomenon in the study area. The most interesting relationships are those that compare the volume of landslide material either with the area of the catchment subtended by the dammed section and with the width of the riverbed–alluvial plain system. The results indicate that as the width of the fluvial system increases, and the area of the river basin subtended by the natural dam increases, the volume of the landslide accumulation necessary to produce occlusion also increases. Only one case, among those examined, escapes this general rule: a landslide of small volume made a full occlusion even though a relatively large area of basin was subtended. In this specific case, landslide velocity played an important role. So, while a high ratio between the volume of the landslide and the area of the affected basin represents a predisposing condition for the occurrence of total occlusion, it is not wholly reliable. Indexes, proposed in the literature, suggesting a quantitative assessment of landslide dam hazard, fail to forecast total occlusion unequivocally. This is because of the special characteristics of each territorial area, from lithological, geomorphological and geological-structural point of views. Such indexes, when they do not lose their intrinsic reliability (as in this case), must at least be identified with, and applied to, specific and restricted territories: they cannot be generalized.