Background. In this paper the authors summarise the primary malignant thyroid tumors (p.m.t.t.) incidence data from their centre over the period 1984–2005. The results are explained in the context of the p.m.t.t. incidence data from the National Cancer Register (1996–2002). Methods and Results. Overall, 6434 thyroid operations were indicated and carried out during the sampling period at the authors’ institution, of which 365 cases were histologically confirmed p.m.t.t. (5,7 % of all histological findings). Specifically, p.m.t.t. were diagnosed in 295 women (80,8 %) and 70 men (19,2 %), with the most frequent occurrence in the 5th decade and a higher recent incidence in younger age groups. Whereas follicular thyroid carcinomas predominated in the 1984–1989 period (24 cases or 66,7 %), the papillary thyroid carcinoma was the most frequent type in 1990–1997 and 1998- 2005: 70 (61,4 %) and 160 (74,4 %) cases respectively. In the context of the national incidence, the authors’ institution increasingly contributes to the diagnosis of p.m.t.t. in Slovakia (8,9 % in 1996 vs. 13,9 % in 2002). However, neither the progress in diagnostic methods nor the capacity expansion for patient examination can explain the increasing incidence of the p.m.t.t. (3,9/106 in 1996 vs. 5,6/106 in 2002 in women and 1,3/106 in 1996 vs. 1,7/106 in 2002 in men, respectively). Conclusions: It remains to be explained why there are significant differences in p.m.t.t. incidence among individual counties. For example, in county Trnava 0,6/106 in men in 1996 vs. 1,5/106 in 2002; in county Prešov 1,2/106 in 1996 vs. 2,4/106 in 2002. Similarly, in women in county Trnava 3,6/106 in 1996 vs. 5,6/106 in 2002 and in county Prešov 5,4/106 in 1996 vs. 8,4/106 in 2002. As far as the districts of Slovakia are concerned, data relative to the male population in 2002 indicate zero incidence in 42 out of 79 districts, with the highest incidence (15,9/106) recorded in Svidník. Likewise, data relative to the female population in 2002 indicate zero incidence in 15 out of 79 districts, with the highest incidence (26,5/106) in Stropkov. These differences call for further (radio)epidemiological, geographical and genetic evaluation.

        Key words: primary malignant thyroid tumor, thyroid cancer, histological type, incidence, cancer register, secular trend.