A diagnosis of congenital goitre was confirmed histologically in piglets which were born hairless and swollen, and with significantly enlarged thyroid glands. The iodine content of the thyroid glands and the serum total thyroxine concentrations were very low. No evidence was found of iodine deficiency or significant goitrogenic activity in the diet fed to the sows. An investigation of the parentage of six affected litters revealed that they all had one or other of two boars as the sire, grandsire or great grandsire. The suspicion of an inherited disorder was confirmed when a test mating of a suspect carrier boar and sow resulted in the birth of two affected piglets. The ratio of the numbers of affected and unaffected piglets was statistically consistent with an autosomal recessive mode of inheritance.