The occurrence of aneuploidy in 108 normal, benign and malignant thyroid tissues was evaluated by DNA flow cytometry. Aneuploid cell populations were found in 60% of the papillary carcinomas, 67% of the follicular carcinomas, 52% of the follicular adenomas and in 10% of the multinodular goiters. None of the non-neoplastic thyroid lesions (chronic thyroiditis and Graves' disease) showed aneuploidy. No significant differences in frequency of aneuploidy and DNA index were found between the well-differentiated and trabecular types of the papillary carcinomas (53%; 1.29 +/- 0.10 vs 75%; 1.30 +/- 0.10, respectively), although the latter type of papillary carcinomas have been shown to be associated with a more aggressive clinical behavior. Also no differences in frequency of aneuploidy were noted between the follicular carcinomas and the follicular adenomas, but the DNA index was significantly higher in the former. These findings indicate that flow cytometric analysis of DNA content may not be helpful in the evaluation of the clinical behavior of papillary carcinomas or in the determination of malignancy of follicular tumors, but may be useful in differentiating neoplastic from non-neoplastic thyroid lesions.