Microspectrophotometric measurement of nuclear DNA content was made on archival smears of fine-needle aspirates from 23 oxyphil thyroid neoplasms. Fourteen tumors were considered benign as judged from the histologic picture as well as follow-up for 7 to 18 years after the operation. Nine tumors were malignant; five of these showed capsular penetration and/or blood vessel invasion as the only signs of malignancy, whereas the remaining four in addition had histologically verified metastases and were the cause of death. The DNA patterns found--diploid, polyploid or aneuploid--appeared to have a limited diagnostic value, since malignancy could not be excluded on this basis. A practically useful finding was, however, that aneuploidy appeared to be associated with a high probability of invasive growth. As regards prognostic information, it was found that euploid patterns occurred in tumors from patients with long survival after surgical treatment, while tumors with aneuploid patterns showed a variable clinical course.