This paper embodies the findings in all proven cases of diphtheria admitted to the Hull City Hospital during the years 1938–43 inclusive. All cases during those 6 years were typed and the clinical and bacteriological findings correlated. Since 1932 all severe cases had been typed and in the winter of 1932–3 a consecutive series of 313 cases was investigated and the results published (Leete, McLeod & Morrison, 1933). The 1933 paper showed a high gravis incidence of 59% and an associated high case fatality rate. Not until the end of 1937 was it again possible to type every case admitted but since then we have done so, and the series now presented embodies observations on 2039 infections, the great majority of which were clinical cases.