Abstract
Chronic transmural inflammatory bowel disease was first described in Scotland by Dalziel in 1913. A study has been made of new cases arising in the Aberdeen area during the 21 years, 1955--75, and is continuing. The number of new cases developing each year rose from 1.2 per 100 000 population at the beginning of the study to a peak of 4.5 in 1969. Thereafter it gradually declined, the mean incidence during the three year period 1973--75 being only 2.6 per 100 000. The decline has affected both males and females. It has been more marked in city dwellers than in the rural population. The former preponderance of city cases has almost disappeared. Disease apparently confined to the small intestine is less common than in 1955, when it constituted 77% of the cases recorded. Today the large intestine or combined forms make up 75% of the new cases which are seen. There is no obvious explanation for the fall in incidence or change in site.