This paper presents the first comprehensive morphometry analysis of normal thyroid, adenomas, and follicular and papillary carcinomas. The mean nuclear volume and the mean nuclear surface increased, while the volume densities of rough endoplasmic reticulum and dense bodies decreased from normal thyroid through adenomas and follicular carcinomas to papillary carcinomas. The different amounts of cytoplasmic organelles probably are related to endocrine function rather than to malignant potential. Papillary carcinomas with a predominantly follicular growth pattern are related more closely to follicular carcinomas than to papillary carcinomas dominated by papillae. This probably indicates a more active endocrine function rather than a different degree of malignancy. Papillary carcinomas, where follicular structures dominate, are therefore expected to respond to radioiodine treatment more favorably than those mainly forming papillae. Papillary carcinomas with abundant ground glass nuclei do not seem to have a lower volume density of heterochromatin than tumors lacking this nuclear feature. In the individual problem case with a follicular tumor, electron microscopy and morphometry cannot even distinguish between an adenoma and a well-differentiated follicular carcinoma.