HindIII restriction enzyme digests of genomic DNA from nine distinct strains of Clostridium difficile were undertaken, and the results were related to those of a previously established typing method based on [35S]methionine-labeled protein profiles. Each of the typed strains identified by its protein profile could also be distinguished by its unique DNA digestion pattern. Analysis of strains isolated from 10 patients during a hospital outbreak of antibiotic-associated colitis revealed identical DNA profiles, confirming a single strain as the source of cross-infection. Characterization of isolates from worldwide sources revealed similar digestion patterns within the same strain type. Restriction endonuclease DNA analysis provides a sensitive and useful technique for studying the epidemiology of C. difficile.