Documented thyrotoxicosis developed in a 12 year old girl with chronic autoimmune thyroiditis. During the following 3 years there was a spontaneous progression from hyperthyroidism to hypothyroidism and vice versa fluctuating from one metabolic state to another. The diagnosis of Hashimoto's thyroiditis was based upon thyroid function tests, elevated titres of antibodies against thyroid constituents, and upon thin-needle biopsy of the enlarged thyroid gland.