The aim of this study was to carry out histomorphological and immunohistochemical analysis of thyroid C cells in 30 patients with simple goitre and hyperactive goitre including Graves-Basedow (G-B) disease, treated surgically. Four tissue samples were always taken from the same internal parts of the gland where the number of C cells in physiological conditions was the highest. C cells were detected in paraffin sections after impregnation with silver salt (Grimelius method) or immunohistochemically, with antibodies against calcitonin, synaptophysin, chromogranin A and neuron-specific enolase. Distinctly less numerous C cells were found in simple and hyperactive goitre than in normal thyroid parenchyma. The majority of C cells showed weak intensity of the examined immunohistochemical reactions. C cells were not observed at all in the texture of nodules well separated by the connective tissue. Proliferative changes concerned only follicular cells.