In the period from 1963 to 1975 data were collected on 1,781 isolates of Y. enterocolitica from human infections in Belgium. There was an uneven spread of the cases over the country, but this could simply reflect the distribution and activity of medical laboratories. Serotype 3 dominated (89.2%) followed by serotype 9 (8.6%). Less than 1% of all isolates were from nonenteral sources. Gastroenteritis was the diagnosis in 86.8% of patients, while 10.6% were suffering from an appendicitis-like syndrome. Gastroenteritis had its peak incidence in the under 5 year group, whereas the pseudo-appendicular syndrome was more frequent in the 10--19 years age group. There was a distinct seasonal peak in late autumn. The significant association with Salmonella isolation from the stool of the same patient has been confirmed.