The simultaneous occurrence of acromegaly, hyperthyroidism and hyperparathyroidism is extremely rare and even that of acromegaly and hyperparathyroidism very unusual. Chromophobe adenomata producing either acromegaly, Cushing's syndrome or hyperthyroidism have been described in a limited number of cases. A unique case of a patient with acromegaly, hyperthyroidism and hyperparathyroidism and a chromophobe pituitary adenoma was studied. In the blood an unusually high level of thyrotrophin (TSH) was found (about 2 mU/ml) but no »long-acting thyroid stimulator« (LATS). The TSH activity remained unaffected by thyroidectomy and by treatment with triiodothyronine but was not found in the blood after surgical hypophysectomy. The pituitary tumour contained TSH only about 20 mU/209 mg weight of tissue, i. e. less than is found in normal human pituitary glands. There was evidence of inadequate secretion of corticotrophin and gonadotrophins. The evidence suggests that both the acromegaly and the hyperthyroidism were caused by increased production of growth hormone (STH) and TSH by the pituitary tumour. The hyperparathyroidism was due to a parathyroid adenoma. In four other acromegalic but euthyroid patients no TSH or LATS activity could be detected in the blood. In these patients the protein-bound iodine (PBI), free thyroxine and triiodothyronine-Sephadex uptake tests were normal. In addition in one of these patients, hyperparathyroidisni due to a parathyroid adenoma was observed. The occurrence of a parathyroid adenoma in acromegaly may be one facet of the multiple endocrine adenoma syndrome. In all 5 subjects the hydroxyproline in serum and urine was increased.