Thorny excrescences are the postsynaptic components of synapses between mossy fibers of granule cells and dendrites of CA3 pyramidal neurons in the hippocampal formation. Very little quantitative data on the number and distribution of excrescences in adult rats are available because, first, the vast majority are grouped into clusters and it is not possible to identify single excrescences within these clusters at the light microscope level. Second, clusters are of varying lengths and are distributed over hundreds of micrometers, making ultrastructural analysis prohibitively time-consuming. Here, by using three-dimensional analysis techniques at the light microscope level, we quantified the number, length, and distribution of excrescence clusters on proximal and midfield pyramidal neurons in the rat. Results indicated that proximal neurons had similar numbers of clusters on their apical and basal trees, and that cluster length was also similar. In contrast, midfield neurons had more apical than basal clusters, and apical clusters were longer. For neurons in both regions, basal clusters were located about 50% closer to somata. Overall, proximal neurons had more clusters than did midfield neurons, but the clusters were shorter; thus, proximal and midfield neurons had about the same total cluster length, and presumably the same number of single excrescences. Based on these data and on published ultrastructural measurements of single excrescences, we estimated an average of 41 excrescences/neuron, and suggest that a pyramidal neuron can be contacted by a maximum of 41 mossy fiber boutons, each from a different granule cell.