Two cases of TSH-secreting pituitary adenoma were reported. Endocrinological and immunohistochemical features of these cases were described and problems in diagnosis and treatment of the rare disease are discussed. [case 1] A 28 year-old woman suffered from hyperthyroidism with a relatively high value of serum TSH (T3; 350 ng/dl, T4; 10.0 micrograms/dl, TSH; 24.5 microU/ml). She was treated with antithyroid drug and then underwent subtotal thyroidectomy. Although the levels of serum T3 and T4 were lowered to within normal range, the level of serum TSH still remained high. One month later, she developed frontal headache, amenorrhea and bitemporal hemianopsia. A CT scan showed an enhanced mass in the sellar and suprasellar region. Preoperative endocrinological studies showed elevated values of TSH (47 microU/ml) and its alpha-subunit (9.0 ng/ml). The levels of both T3 (190 ng/dl) and T4 (10.0 micrograms/dl) were near the upper normal limit. Serum TSH was suppressed by administration of exogenous T3, but did not respond to exogenous TRH, l-Dopa nor bromocriptine. Under the diagnosis of TSH-secreting pituitary adenoma, the patient was operated on by craniotomy and received local radiation therapy (50 Gy). In 1990, 12 years after the treatment, she is well and endocrinologically normal. Immunohistochemical study revealed that most tumor cells were positive for TSH. [case 2] A 28 year-old woman visited our hospital for examination of hyperthyroidism. Serum level of TSH was detectable (4.5 microU/ml). A CT scan performed at that time disclosed no pituitary tumor. Thyroid function was normalized by antithyroid drug, but the level of TSH was still high and progressively increased.(ABSTRACT TRUNCATED AT 250 WORDS)