Although diphtheria is declining in Delhi, case fatality rates (CFRs) are rising. In 1997, of 143 clinically suspected cases admitted to the Infectious Diseases Hospital 45 (32%) died. We examined their records to understand the epidemiology and reasons for high CFRs. About 53% of cases were from Delhi; they were not limited to any particular area. All the deaths and 92% (131/143) of cases occurred in children below 10 years of age. Only 12% of cases had received one or more doses of DPT. Muslims contributed significantly more cases than Hindus. CFRs were significantly higher in young (P=0·03) and unvaccinated (P=0·01) children and in those who received antitoxin on the third day of illness or later (P=0·03). The study highlights the importance of improved vaccine coverage and early diagnosis and prompt administration of antitoxin in reducing CFRs for diphtheria in Delhi.