In a series of 420 autopsies the thyroid glands have been weighed, serial sections made and examined histologically with at least one specimen per lobe. There is an age-dependent increase in mean thyroid weight. In the 7th decade the mean weight exceeds 29 g and weights above this are regarded as goitre. Between the 4th and 8th decade the incidence of goitre is 20-30%, after which the incidence rises steadily to 64% in the 10th decade. The commonest thyroid pathology is hyperplastic nodules, which are found in 39% of patients (49.4% of males and 33.4% of females). 2.4% of all patients have adenomas, 1.9% primary carcinomas and 2.8% thyroid metastases. Inflammatory infiltrates are observed in 6.6% of cases (9.4% of females and 4.4 of males). To establish the incidence of normal thyroid glands we have studied 840 serial autopsies. Only 25% of thyroids are normal (19% of females and 30% of males). The incidence of normal thyroid glands tends to decrease with age and in no decade does it reach 50%.