A group of related borzoi dogs were studied over a 6-year period to ascertain the cause of primary hypothyroidism. Four generations of dogs were analyzed. Two littermates with lymphocytic thyroiditis were mated and the 10 offspring were all diagnosed, on the basis of thyroid biopsy evaluation, as having lymphocytic thyroiditis by age 2.5 years. A wide range of thyroid gland lesions was demonstrated in this litter of dogs. This report documents the occurrence of lymphocytic thyroiditis in three successive generations of an inbred group of borzoi dogs. An autosomal recessive mode of inheritance for the trait in this group of dogs is proposed.