Rock masses detached as rockfalls usually disintegrate upon impact on the ground surface. The Rockfall Block Size Distribution (RBSD) generated by the propagation of the rockfall mass is required for the analysis of the trajectories of the blocks, the run-out distances, the impact energies, the quantitative assessment of the rockfall hazard and for the understanding of the fragmentation process. We have measured the volume of the blocks detached in 5 rockfall cases, obtaining the corresponding RBSD. The total volume involved in these rockfall events ranges from 2.6 m3 to 10000 m3. The obtained RBSD can be well fitted by power laws with exponents ranging from 0.51 to 1.27. The results suggest that these exponents may be related to the height of fall (potential energy) and to the proportion of new fractures generated in the rock mass, among other factors.