This article highlights the prevalence of diphtheria in urban slums in north India. In 1997, diphtheria cases reached 1326, and in a study conducted in September 1999, 4 diphtheria cases were reported in a tertiary care hospital. These cases occurred despite the vaccination programs against diphtheria. Clinical manifestation included fever, sore throat, dysphagia, and swelling in the neck of 2-10 days duration. Immunization records of these children revealed that 2 were nonimmunized and the other 2 received only 2 doses of diphtheria-pertussis-tetanus vaccine in their first year of life. Using electrocardiography, 3 of the cases were suggestive of myocarditis. Laboratory examinations, such as Albert's staining and Loeffler's serum slope, were used to confirm the diagnosis. This epidemic was found to be caused by a large population of susceptible children and adults, decline in childhood immunization, poor socioeconomic conditions, and large-scale population movements. In conclusion, the 44% dropout rate between the third dose of primary immunization and the first booster could be one factor in the emergence of diphtheria.