A 38-yr-old male with acquired immunodeficiency syndrome developed neck pain in association with mild hyperthyroidism and a diffusely enlarged thyroid gland. Radioactive iodine scanning was consistent with thyroiditis, and biopsy of the thyroid demonstrated the presence of pneumocystis carinii in thyroid tissue. Treatment with pentamidine followed by trimethoprim sulfamethoxazole led to rapid normalization of thyroid size in association with the development of hypothyroidism. This case illustrates the natural history of pneumocystis carinii thyroiditis and suggests that thyroid disease be added to the spectrum of human immunodeficiency virus-associated endocrine dysfunction.