The new (2004) WHO Classification of Tumours of the Thyroid and Parathyroid Gland exhibits both formal and contentual changes compared to the previous one from the year 1988. Comparing both texts we have tried to apply the new norm to some recent diagnostic tasks. The classification of poorly differentiated and undifferentiated neoplasms has been successfully solved by employing the immunohistochemical procedures into the daily routine. Nevertheless, some diagnostic problems persist and can not be solved this way. This is true especially for the borderline neoplasm in the FVPTC category and even more for its oncocytic variant. There appeared shift in the criteria for the diagnosis of oncocytic variant of papillary carcinoma. Newly, the obligatory presence of the nuclei with the characteristics of the conventional papillary neoplasms is required. Thorough elaboration of the TNM system has solved the problem of multifocal lesion staging. Moreover, a web consultation source is recommended. The new Classification includes descriptions of the basic cytodiagnostic features. The post aspiration capsular damage consideration in relation to the diagnosis of the minimally invasive follicular carcinoma is one practical profit thereof. In terms of formal logic a criticisable feature of the new classification is the introduction of the so called „Synonyms“. These listings include in fact partly real synonyms by definition, but together in one table quite often also subunits or variants of the nosologic units described. In general, the new Classification represents a successful consensus reached. As there are newly also references with varying opinions included, it is presented more like a point of gravity in this field than an undoubted truth. This also makes it a valuable and useful normative text.

        Key words: thyroid – thyroid neoplasms – WHO classification of tumours – diagnostic pitfalls – synonyms