Background Tumour-infiltrating lymphocytes (TILs) are a protective prognostic factor in several solid tumours and predict response to immune checkpoint inhibitor therapy. The prognostic impact of TILs in Medullary Thyroid Cancer (MTC) is poorly understood. Methods Using the International TILs Working Group System (ITWG) we assessed the TILs profile of primary MTC tumours and correlated this with clinicopathological prognostic variables, including the International Medullary Thyroid Cancer Grading System (IMTCGS) grade and survival outcomes. Results Using the ITWG system, all patients with MTC had low TILs, with a median (range) of 3% (0-10%). This group was further subdivided into ‘very low’ 0-4% and ‘low’ 5-10%, and on cox regression analysis, increasing TILs were associated with increased local recurrence (log rank p=0.022), reduced disease-specific survival (log rank p=0.015) and a trend to decreased distant metastasis-free survival (log rank p=0.14). When examining the association between TILs and other prognostic factors, only high IMTCGS grade was associated with increased TILs. On multivariable logistic regression analysis, there was no significant association between TILs and local recurrence or disease-specific survival. Conclusions Our study demonstrated that TILs are not prognostic in MTC. Even high-grade MTC can be considered an immune quiescent tumour, and the adverse prognostic factors associated with higher grade tumours outweigh the marginal increase in immune recognition associated with a slight increase in TILs. The low level of TILs in MTC and their lack of correlation with survival suggest that immune checkpoint inhibitor therapy may not be effective.