Most patients with thyroid cancer will evolve very well with current therapies. However, 10-30% of these patients will present recurrent disease and some of them will eventually die. IL-10 is an anti-inflammatory and immunosuppressive cytokine that can contribute to the immune escape of neoplastic cells. We aimed to investigate IL-10 as a molecular marker to improve the clinical management of patients with thyroid cancer. We retrospectively studied 162 patients with follicular cell-derived thyroid cancer who attended to our institution, including 63 classic papillary thyroid carcinomas, 46 follicular variant of papillary thyroid carcinomas, 11 poorly differentiated thyroid carcinomas and 42 follicular thyroid carcinomas. Patients were treated according to current guidelines and followed-up for 1-150 months. Additionally, we studied 96 samples of non-malignant tissues. We investigated the expression of IL-10 in tumor cells by semiquantitative and quantitative methods. Malignant tissues presented higher positivity (0.773 +/- 0.140) than non-malignant samples (0.623 +/- 0.190; p < 0.001). Tumors with extrathyroidal invasion at diagnosis presented higher levels of positivity for IL-10 (0.802 +/- 0.125) than tumors without extrathyroidal invasion (0.731 +/- 0.147; p = 0.004). We observed a positive correlation between tumor size and IL-10 positivity (correlation coefficient = 0.407; p < 0.001). Patients with IL-10 positivity above the median presented lower relapse-free survival rate compared to those patients whose tumors presented IL-10 positivity below the median. We suggest that a simple IL-10 IHC analysis could help selecting patients who would benefit from a more intensive approach.