A case of autoimmune thyroiditis after long term treatment with lithium is described in a 29 year old Japanese woman with manic depression. Positive serum antithyroglobulin and antimicrosomal antibodies, diffuse goitre, and microscopic chronic thyroiditis, as well as the clinical history of long term lithium treatment were suggestive of lithium associated autoimmune thyroiditis. Microscopically, there was a mild degree of interstitial fibrosis and a moderate degree of lymphocytic infiltration. Some areas showed a moderate degree of stromal fibrosis and atrophic thyroid follicles. Lymphoid follicles with germinal centres, disrupted thyroid follicles with lymphocytic infiltration, and Hürthle cells were also observed. The differential diagnosis in patients presenting with these histological features includes painless (silent) thyroiditis, autoimmune thyroiditis and lithium associated autoimmune thyroiditis. A detailed clinical history is essential if the correct diagnosis is to be reached.