During the 18-year (1974-1991) period the lymphocytic thyroiditis was found in 8% of 2238 animals from a Beagle dog breeding colony (8.5% of 1079 males and 7.6% of 1159 females). The thyroid glands of affected animals were on average heavier by 10.4% in males and by 21.0% in females. In the histological picture the interstitial lymphocytic infiltrations of various ranges accompanied by varying degree of follicular damage prevailed. The earliest findings of this lesion were revealed in 9-month old animals.