Subacute thyroiditis is a frequent benign thyroid disease associated with previous viral upper respiratory tract infection. Known complications of this disease are long-standing subclinical hypothyroidism, persistent anterior neck pain and rarely Graves disease. In general, thyroid abscess is an uncommon disease because of anatomic isolation of the gland and its rich system of drainage for blood and lymph. Especially, development of thyroid abscess in subacute thyroiditis is extremely rare phenomenan, but significant bad outcomes can be resulted. Its clinical BACKGROUND containes immune-suppressed state, anatomic defect, presence of underlying other thyroid disease and of non-thyroidal infectious foci. We experienced a case of subacute thyroiditis complicated with streptococcal thyroid abscess during glucocorticoid therapy. The patient was a 19-year-old female who was admitted due to anterior neck pain for 1 month. Typical subacute thyroiditis was suggested from initial laboratory findings including CBC, erythrocyte sedimentation rate, serum T3, T4, TSH levels, thyroid scan & thyroid uptake. But during oral prednisolone therapy, unexpected bacterial thyroid abscess was developed. We report this unusual case with review of literatures.