This study addressed the immunohistochemical expression of MUC1 in papillary thyroid carcinoma (PTC) of different histotypes, sizes, and morphological features of aggressiveness, and its correlation with the overexpression of cyclin D1, a target molecule of the Wnt pathway. MUC1 expression was examined in a total of 209 PTCs. Cytoplasmic MUC1 expression was elevated in the tall, columnar cell and oncocytic variants (100%), Warthin-like (78%), and conventional PTCs (61%), and in papillary microcarcinoma (PMC) with the conventional growth pattern (52%). On the contrary, it was low in the follicular variant (27%) of PTC and PMCs with follicular architecture (13%). Cytoplasmic MUC1 accumulation did not associate with any clinicopathological features except peritumoral lymphoid infiltration in PTCs and in PMCs with the conventional growth pattern. MUC1 staining correlated with cyclin D1 overexpression in conventional PTCs and PMCs and PMCs with follicular architecture. The results demonstrate that MUC1 expression varies broadly in different histological variants of PTC, being the lowest in tumors with follicular structure. In general, it does not prove to be a prognosticator of PTC aggressiveness. A high correlation between MUC1 and cyclin D1 implies MUC1 involvement in the Wnt cascade functioning in a large subset of human PTCs and PMCs.