Tumor-infiltrating lymphocytes (TILs) are associated with improved survival in several types of cancers, including genitourinary cancers. However, multiple different scoring methods used to assess TILs complicate the comparison of different studies and are not always suitable for daily practice. In 2014, the International TILs Working Group (ITWG) proposed a simple and robust assessment method for a more standardized evaluation of TILs. Here, we validated this system in muscle-invasive urinary bladder cancer (MIBC). Patient history and histologic specimens from 203 patients with MIBC were retrospectively analyzed. The stromal TIL (sTIL) score was determined using the ITWG system and 3 groups were defined according to the degree of stromal lymphocytic infiltration: low (0-10%), intermediate (10-55%) and high (55-100%). Associations between sTIL score, clinicopathological variables, tumor-specific survival (TSS), overall survival (OS), and disease-free survival (DFS) were analyzed. High stromal lymphocytic infiltration was associated with significantly higher OS, TSS and DFS when compared to low grade sTILs. The survival benefit remained statistically significant in multivariate analyses, confirming that sTILs are a strong independent positive prognostic factor in patients with MIBC. In summary, the degree of sTILs as defined by the ITWG robustly predicts survival in MIBC patients. Prospective studies with larger case numbers are needed to determine whether sTILs should be included in staging guidelines and how they could aid in therapeutic decision making.