Summary: Scaffold network generator (SNG) is an open-source command- line utility that computes the hierarchical network of scaffolds that define a large set of input molecules. Scaffold networks are useful for visualizing, analysing and understanding the chemical data that is increasingly available through large public repositories like PubChem. For example, some groups have used scaffold networks to identify missed-actives in high-throughput screens of small molecules with bioassays. Substantially improving on existing software, SNG is robust enough to work on millions of molecules at a time with a simple command-line interface. Availability and implementation: SNG is accessible at http://swami.wustl.edu/sng Contact: firstname.lastname@example.org Supplementary information: Supplementary data are available at Bioinformatics online.