Rock masses detached as rockfalls usually disintegrate upon impact on the ground surface. The knowledge of the Rockfall Block Size Distribution (RBSD) generated by the propagation of the rockfall mass is required for th e analysis of the trajectories o f the blocks, the run - out distances, the impac t energies, the quantitative assessment of the rockfall hazard and for the understanding of the fra g- mentation process . We have measured the volume of the blocks detached in 5 rockfall cases, obtaining the corresponding RBSD. The total volume involved in th ese rockfall events ranges from 2.6 m 3 to 10000 m 3 . The obtained RBS D can be well fitted by power laws with exp o- nents ranging from 0.51 to 1.27. The results suggest that t hese exponents may be related to the height of fall ( potential energy ) and to the pro portion of new fractures generated in the rock mass , among other factors.