A retrospective study of the pathology seen in 1,000 consecutive appendicectomy specimens with a pre-operative diagnosis of acute appendicitis, was made in an attempt to find common factors which might throw light on the aetiology of the disease. Acute inflammation was seen in 775 cases. In 225 cases where no inflammation was seen, 168 appendices were associated with other pathology, i.e., lymphoid hyperplasia, fibrosis and tumour. In 57 cases the appendix was not inflamed. Acute inflammation was seen more in males and in patients aged below 30 years, while normal appendices were more commonly seen in females of reproductive age.