Viremia with turkey herpesvirus (HVT) and/or Marek's disease (MD) herpesvirus (MDHV) was examined in chickens from three different commercial flocks experiencing "vaccine breaks" following vaccination with HVT vaccine against MD. In all groups of sample chickens from the flocks, the incidence of detectable HVT viremia was significantly less in the MD-affected than in the healthy penmates, and incidence of MD was significantly higher in the birds without detectable HVT viremia than in the viremic penmates. The results suggest an association between HVT viremia and protection against MD development.