Twenty patients with autoimmune endocrinopathies experienced 45 episodes of pleural and/or pericardial serositis. Seventeen of these patients were women and 15 had clinical or serologic evidence of 2 or more endocrinopathies. Idiopathic primary hypoadrenalism (10 cases), Graves' disease (8 cases), Hashimoto's disease (4 cases), atrophic thyroiditis with hypothyroidism (3 cases), idiopathic primary hypogonadism (3 cases), transient thyroiditides (2 cases), and type I diabetes mellitus (1 case) were diagnosed at a mean age of 24 years. Serositis recurred after asymptomatic intervals of months to years even in patients treated for endocrine dysfunction. Fourteen of 16 Caucasians had circulating immune complexes, including all 9 patients with a C4AQ0 (C4A null) phenotype and including all 12 patients with HLA antigens B8 and DR3, antigens associated with systemic lupus and with autoimmune endocrinopathies. Serositides associated with autoimmune endocrinopathies can occur with chest pain, fever, and exudative effusions in young Caucasian women with the HLA B8 DR3 C4AQ0 phenotype. These serositides may have a common pathophysiologic mechanism.