Since 1990, there has been a major resurgence of diphtheria in Europe, centered mostly in Russia and Ukraine. The outbreak in the Russian Federation is the largest diphtheria outbreak in the developed world in recent years. In 1990, 1211 cases were reported, of which one-third were from Moscow. Outbreaks continued in 1991 (1869 reported cases) and 1992 (3897 reported cases). In 1993, the epidemic drastically increased in magnitude (15,210 reported cases) [1]. In the first 2 months of 1994, M000 cases were reported. From 8 February to 4 March, 525 patients with diphtheria were treated in Moscow hospitals. Most persons infected are ^4 years old, and ^S^c of cases have been bacteriologically confirmed. The case-fatality ratio remains low ^5(^) [1]. Since initial studies verified the effectiveness of the vaccines in use in Russia and Ukraine, the reasons for the outbreak remain unclear and are under investigation. High levels of susceptibility to diphtheria, particularly among adults, have probably contributed most strongly to the epidemic. In addition to Ukraine (1991-1994, 6000 cases), other countries that have experienced outbreaks of diphtheria in recent years include China (1988-1989, 103 cases), Tajikistan (1993, 1100 cases), Ecuador (1993-1994, several hundred cases), and Algeria. Because of concern about diphtheria worldwide and the level of US preparedness, the National Vaccine Program held a Diphtheria Diagnostic Workshop hosted by the Centers for Disease Control and Prevention (CDC). Workshop goals were to review the current state of diphtheria diagnostics, assess the capability of US laboratories to diagnose diphtheria, and identify directions for future research and preparedness activities. Twenty-three persons from the United States, United Kingdom, Russia, Italy, and Finland met to discuss issues related to diagnosis of diphtheria, share expertise, and determine areas of mutual support. Dr. Melinda Wharton (National Immunization Program, CDC) presented information on the current epidemiology of diphtheria in the United States. In contrast to the situation early in this century, when diphtheria was a major cause of