Previous to 1926, the type of diphtheria seen in Vienna was mild, but since that year there has been a great deal of severe diphtheria, with numerous complications and high fatality. The author reports his clinical observation of numerous cases of malignant diphtheria. The clinical course, pathology and complications are discussed extensively. The epidemiology, prophylaxis and treatment are also considered. Comparison is made with milder forms of diphtheria seen in other years. This is an interesting and informative presentation of the type of diphtheria seen in Vienna during the past few years.