To find reliable biomarkers for high-grade malignancy, the relationship between immunohistochemical L-type amino-acid transporter 1 (LAT1) expression of biopsy samples, determined with the newly developed monoclonal antibody against human LAT1, and prognosis of patients with prostate cancer, was investigated. The intensity and score of immunohistochemical LAT1 expression of first biopsy samples were assessed using the modified Sinicrope et al. method and were found to be correlated with poor survival for the study group of 114 surgically treated patients as a whole (P = 0.0002 and 0.0270, respectively). LAT1 intensity further had a significant relationship (P = 0.0057) with prognosis in pathological T3 + T4 groups. Multivariate analysis indicated that the LAT1 intensity and score were more reliable prognostic markers, compared with the Gleason score and the Ki-67 labeling index. A relationship of the LAT1 intensity and score with prognosis could also be confirmed in 63 patients with inoperable cancer (P = 0.0070 and <0.0001, respectively). Similarly, significant differences in prognosis were confirmed in clinical T3 + T4 groups (P = 0.0091 and 0.0244, respectively). Moreover, the combination of LAT1 expression and Gleason score was found to have a more reliable correlation with prognosis. Thus, elevated LAT1 expression in prostate cancers is a novel independent biomarker of high-grade malignancy, which can be utilized together with the Gleason score, which is mainly dependent on cellular and structural atypia, to assess prognosis.