Background: Patients with differentiated thyroid cancer (DTC) usually have a good prognosis but may experience a decline in quality of life (QOL). The way patients perceive their illness may have a major impact on their QOL. Our hypothesis was that patients with DTC frequently perceive their illness as much more severe than its objective clinical characteristics indicate. The aim of the study was to investigate how patients with DTC perceive their illness and to correlate these findings to various demographic parameters as well as objective indices of disease severity. Methods: The self-administered Illness Perception Questionnaire-Revised (IPQ-R) was completed by consecutive patients with DTC during routine follow-up at the endocrine clinic. The questionnaire consists of three parts that measure different aspects of illness perception. The patients' medical records were reviewed for data on demographic parameters (sex, age) and indices of disease severity (duration of DTC, disease stage at diagnosis, number of operations, number of radioactive iodine treatments, and evidence of disease persistence/recurrence). The patients were also asked for additional data on family status, level of education, and employment status. Pearson and Spearman correlations and analysis of variance were used for statistical analysis. Results: The study group included 110 patients (91 women) of mean age 53.5 years. Level of education was the only demographic factor found to affect the patients' perception of their illness. There was no correlation of patient illness perception and cancer stage. Among the disease-severity parameters, time since last treatment, evidence of disease persistence, and number of iodine treatments were significantly associated with a negative disease perception. Number of iodine treatments was the most broadly affecting factor. There was a high correlation of scores among the various illness perception subscales. Conclusions: Patients with DTC perceive their illness on a subjective, emotional basis unrelated to its actual severity. To improve patients' illness representations and, consequently, their QOL, a trained psychologist should be included in the multidisciplinary team that manages patients with DTC. Attention should be particularly directed to less-educated patients and patients who require repeated iodine treatments.