An outbreak of respiratory diphtheria occurred among highly-vaccinated trainees at a Latvian military academy in August-September 2000. We reviewed immunization, clinical and laboratory records and administered a questionnaire to obtain data on exposure factors. Among 207 trainees, 45 (22%) diphtheria cases and 79 (38%) carriers of toxigenic Corynebacterium diphtheriae were identified. All patients survived; 1 had severe myocarditis. Sharing cups was a risk factor for infection. Over 85% of trainees had received > or =5 doses of diphtheria toxoid. Neither infection nor disease was associated with the number of doses or interval since last dose. However, the risk of disease was lower and diphtheria antitoxin levels were higher among trainees who received their last booster dose with higher-antigen diphtheria toxoid (DT) instead of lower-antigen Td. Outbreaks of mild diphtheria can occur among highly-vaccinated persons living in crowded conditions with intense exposure; high-antigen diphtheria booster-vaccination might provide better protection under these conditions.