Following the introduction of diphtheria toxoid in Thailand in 1978 and the acceleration of vaccination efforts by the Expanded Program on Immunization (EPI), the incidence of diphtheria in Thailand decreased substantially. A total of 1021 diphtheria cases were reported nationwide in 1984, compared with 25 cases in 1993. However, on July 19, 1994, a cluster of cases of diphtheria was reported in Saraburi province (1994 population: 565,067) to the Division of Epidemiology (DOE), Thailand Ministry of Public Health; no cases had been reported during the previous year in Saraburi. This report summarizes the outbreak investigation by the DOE, which identified 18 cases during April-August 1994 and identified carriage rates of Corynebacterium diphtheriae of 4% and 8% among household contacts and school contacts, respectively.