We used pulsed-field gel electrophoresis (PFGE) to study the genetic relatedness of 235 isolates of Shigella flexneri and Shigella sonnei collected in Hong Kong (97 isolates from 1986 and 1987 and 138 isolates from 1994 and 1995). Altogether, 13 gels were run with bacteriophage lambda ladder DNA (Pharmacia) as an external reference in every sixth lane, standardized reagents and methods, and isolates randomized for species and years. For quantitative illustration of the relationships within a large body of isolates, computer-generated dendrograms were used to determine the number of isolates in pulsotypes at Dice coefficients of similarity of 75% (PT₇₅) and 50% (PT₅₀). For S. flexneri, there was a significant difference in the distribution of isolates collected during the two periods in both PT₇₅ and PT₅₀, with 68% of isolates collected in 1994 and 1995 sharing a coefficient of similarity of 68%. For S. sonnei, a significant difference was observed in PT₅₀ only. We also used Upholt's formula for an approximation of the fraction of nucleotide difference between isolates and Molecular Evolutionary Genetics Analysis to determine relative genetic distances. For both species, the relative genetic distances between isolates of the earlier collection period were significantly greater (P < 0.0001), i.e., they were further apart and therefore more diverse than those of the later period. We conclude that it is possible for a typical clinical laboratory to analyze a large amount of PFGE information on Shigella isolates obtained under controlled conditions. Such data analysis should enhance surveillance capabilities and give indications of further work to be done on various aspects of bacterial pathogenicity of the species.