Punch biopsy was carried out in 32 cases in which thyroid gland changes were sonographically classified as being of homogeneously normal or of homogeneously low echogenicity. Mean follicle lumen size was morphometrically determined from the histological sections. This produced a significant, positive correlation between echogenicity and follicle size. The mean follicle lumen diameter in so-called echonormal structures was 67 microns (SD +/- 23 microns), and 25 microns (SD +/- 8 microns) in low-echogenic lesions. Thus normal echogenicity represented a normofollicular or macrofollicular structure, while a low echogenicity pattern indicated a microfollicular or solid tissue structure. The structure of thyroid carcinomas is not homogeneously normofollicular or macrofollicular; in exceptional cases they are such only focally. Therefore the significance of these findings lies in the exclusion of malignancy in the event of homogeneously normal echogenicity.