OBJECTIVES: The association of autoimmune thyroiditis (AIT) with papillary thyroid carcinoma (PTC) has been studied for over 60 years, yet their causal relationship has not been elucidated. Most published papers report a better prognosis of the patients with tumour in the fi eld of thyroiditis. In our work we aimed to fi nd out the differences in the clinical behaviour of PTC depending on the presence of autoimmune infl ammation. METHODS: We retrospectively analysed a group of 1,201 patients with PTC dispensed in St. Elisabeth Cancer Institute and Faculty of Medicine from 2000 to 2015. We divided patients with AIT according to the time of diagnosis of infl ammation into the AIT1 subgroup, which included patients monitored for AIT before tumour detection. In them, we assumed that the factor of long-term endocrinological monitoring could speed up the diagnosis of the tumour and thus improve the prognosis. The AIT2 subgroup consisted of patients with both tumour and infl ammation diagnosed simultaneously, thus eliminating the factor of prior monitoring. RESULTS: PTC in the AIT1 subgroup had better prognostic parameters (TNM stage, persistence, disease remission). Patients in the AIT2 group had all monitored parameters comparable with patients with tumours without autoimmune infl ammation. CONCLUSION: AIT alone does not have a protective effect on the course of PTC, the cause of a better prognosis in the AIT1 subgroup is a different pathomechanism of carcinogenesis, as well as previous endocrinological monitoring and earlier detection of malignancy (Tab. 4, Fig. 2, Ref. 27).