The dissemination of equine herpesvirus types 1 (EHV-1) and 4 (EHV-4) among various horse populations in Japan was investigated through the isolation and typing of virus strains from horses with respiratory diseases. Type specific monoclonal antibody pools were used for the typing of isolates. The 42 strains of EHV-1 and 64 strains of EHV-4 were isolated from 4593 nasal swabs and/or blood plasma samples collected from 3326 horses during a period from 1979 to 1990. All the strains of EHV-1 were isolated from racehorses only and during the winter season exclusively, when the epizootic of respiratory diseases occurred among racehorse populations at two Training Centers of the Japan Racing Association. In contrast, the strains of EHV-4 were isolated from horses irrespective of the season, facility, or horse population; foals and yearlings in breeding farms and our institute, rearing horses in rearing farms, and racehorses. Especially, 4 strains of EHV-4 were isolated from plasma samples containing buffy coat cells. We believe this is the first reported case of EHV-4 cell-associated viremia in the world. All 87 strains isolated from aborted fetuses were identified as EHV-1. The results suggest that EHV-1 is responsible for epizootic respiratory diseases in racehorses in the winter and abortion among mares at the late stage of gestation, and that EHV-4 causes respiratory diseases throughout the year among all horse populations.