Pneumocystis cariniiis the most common cause of opportunistic pulmonary infection in AIDS patients and disseminated disease is being recognized with increasing frequency. We describe a patient with cavitary pulmonary disease, lymphadenopathy, thyroiditis, and associated hypothyroidism, all a result ofP. carinii.The organism was easily demonstrated in a fine-needle aspirate specimen of the thyroid. This is the second reported case of clinically apparentPneumocystisthyroiditis and the first reported case of hypothyroidism due to an opportunistic infection in a patient with AIDS. Clinicians should be aware of this entity and request a Grocott-Gomori methenamine-silver nitrate stain of appropriate cytology specimens to make the diagnosis.