One-hundred and seventy-two epidemiologically unrelated herpes simplex virus type 1 (HSV-1) strains isolated in Japan (104 strains) and Sweden (68 strains) were compared by analysis of their genome structures using four restriction endonucleases, BamHI, KpnI, SalI and HindIII. In addition, 32 Kenyan HSV-1 isolates previously compared to Japanese isolates were included for further comparison with the Swedish isolates. Remarkable and statistically significant differences were found between the HSV-1 isolates from the three countries. One-hundred and thirty cleavage sites were examined, and it was shown that isolates from two of the three countries were statistically distinguishable at 27 of these loci. Pairwise comparison between isolates from Japan and Sweden, Kenya and Sweden, and Japan and Kenya revealed variation in 18, 16 and 23 sites, respectively. By considering gains and losses of 19 sites, the total of 204 strains could be classified into 92 distinct cleavage patterns. Isolates from the three countries could be distinguished from each other by the pattern, except for one Swedish and two Kenyan isolates which shared a pattern. Twenty-one fragments that were present or absent only in individual isolates from one or other of the three countries could be detected. These results show that HSV-1 strains from geographically separate countries or anthropologically different races have distinct distributions of endonuclease recognition sites.