Mucinous carcinoma of the thyroid gland is an uncommon tumour that from the histological point of view, resembles mucinous carcinoma of others sites. Although a mucinous appearance has sometimes been reported in association with cases of typical thyroid carcinoma, true mucinous carcinoma is exceptionally rare. We describe two cases of thyroid tumours with mucinous differentiation studied with immunohistochemistry. Both cases disclosed a similar histological appearance, with small nests and sheets of malignant epithelial cells associated with extensive extracellular mucin that substituted and entrapped the follicular parenchyma of the thyroid. Thyroglobulin and focally thyroid transcription factor (TTF) 1 were positive in one case. From these findings, we classified this tumour as primary mucinous thyroid carcinoma. Thyroglobulin and TTF-1 were negative in tumour cells of second case; on the contrary, positivity to the carcinoembryonic antigen and CA-125 was strong and generalized. However, successfully, the patient presented ascites associated to right ovarian mass. In this case, thyroid tumour represents the first clinical sign of an ovarian mucinous adenocarcinoma, and it has not been previously described in literature. Both patients died after few months to diagnosis. In conclusion, primary and secondary mucinous carcinoma are rare and unusual tumours of the thyroid gland that can be a cause of pitfall in differential diagnosis. In these cases, for a correct diagnosis, a complete clinical history, restricted histological criteria and immunohistochemical panel are necessary.