Isolated publications note the association of thyroid carcinoma, thymoma, and parathyroid adenoma with malignancies of other organs or systems. Of 166 patients with histologically proven parathyroid adenomas, 42% of those found at autopsy and 16% of such patients with parathyroidectomy and no autopsy had such malignancies. Seventy-five similar patients were added to these from literature survey. Gastrointestinal, breast, thyroid, genitourinary, and respiratory systems were the primary sites of malignancy, in decreasing order. These were significantly higher than that observed in control autopsies of comparable age and sex. In contrast, thyroid carcinoma is associated with respiratory, gastrointestinal, breast, and genital malignancy; thymoma with dermatologic, respiratory, thyroid, gastrointestinal, and breast malignancies, both groups in decreasing order of frequency. The pathogenesis and significance of such observations remain obscure in some instances.