We have systematically enumerated graph representations of scaffold topologies for up to 8-ring molecules and 4-valence atoms, thus providing coverage of the lower portion of the chemical space of small molecules (Pollock et al. 1 ). Here, we examine scaffold topology distributions for several databases: ChemNavigator and PubChem for commercially available chemicals, the Dictionary of Natural Products, a set of 2,742 launched drugs, WOMBAT, a database of medicinal chemistry compounds, and two subsets of PubChem, "actives" and DSSTox comprising toxic substances. We also examined a virtual database of exhaustively enumerated small organic molecules, GDB, 2 and contrast the scaffold topology distribution from these collections to the complete coverage of up to 8-ring molecules. For reasons related, perhaps, to synthetic accessibility and complexity, scaffolds exhibiting 6 rings or more are poorly represented. Among all collections examined, PubChem has the greatest scaffold topological diversity, whereas GDB is the most limited. More than 50% of all entries (13,000,000+ actual and 13,000,000+ virtual compounds) exhibit only 8 distinct topologies, one of which is the non-scaffold topology that represents all treelike structures. However, most of the topologies are represented by a single or very small number of examples. Within topologies, we found that 3-way scaffold connections (3-nodes) are much more frequent compared to 4-way (4-node) connections. Fused rings have a slightly higher frequency in biologically oriented databases. Scaffold topologies can be the first step toward an efficient coarse-grained classification scheme of the molecules found in chemical databases.