The usefulness of negative-contrast electron microscopy in the rapid differential diagnosis of poxvirus and herpesvirus exanthems is described in this study of 301 specimens from patients with vesicular exanthematous diseases. Specimens from patients with smallpox, various forms of vaccination complications, varicella, zoster (shingles), and herpes simplex are included in this evaluation. Electron microscopy, when applied to the study of lesion material, was found to be more sensitive than the classical techniques of virus isolation in the diagnosis of both poxvirus and herpes/varicella virus infections. However, since specific identification of a virus within a group cannot be made morphologically by electron microscopy, it is recommended that both electron microscopy and virus isolation methods be employed for the routine differential diagnosis of vesicular exanthematous diseases in the reference diagnostic laboratory.