To establish the incidence and describe the clinical epidemiology of necrobacillosis and Lemierre's syndrome in Denmark, the clinical records of all laboratory-recorded cases of septicaemia due to Fusobacterium necrophorum biovar A, B, and C were reviewed retrospectively during a 6-year period. The incidence of necrobacillosis and Lemierre's syndrome was 1.5 and 0.8 per million persons per year, respectively, showing a tendency to increase during the period. Fusobacterium necrophorum was grown after three days' incubation, but the characteristic pleomorphic fusiform morphology was often disregarded as an important help in diagnosing necrobacillosis. The 24 patients with Lemierre's syndrome were all young and previously healthy, and none died, but pre-hospital delay was associated with a significantly higher morbidity and risk of metastatic infections. The remaining 25 patients with necrobacillosis had a high mortality, 24%, which was correlated with age and predisposing diseases, especially cancers. These findings stress the importance of a quicker clinical and microbiological diagnosis in cases of Lemierre's syndrome, and of screening for cancer in the remaining cases of necrobacillosis.