The Lewis X (Le(X)) antigen is a prognostic marker in certain solid tumors and has been proposed as a therapeutic target. Immunohistochemical analyses were performed to retrospectively examine the prognostic impact of Le(X) expression in 158 patients with triple-negative breast cancer (TNBC). Twenty-three (14.5%) patients were Le(X)-positive. Multivariate analysis showed that Le(X) positivity was an independent poor prognostic factor for recurrence-free survival (RFS) and overall survival (OS) (P = .037 and P = .024, respectively). Le(X) expression was a prognostic factor for survival in stage I/II and stage III TNBCs. Subgroup analysis according to age showed that Le(X) positivity was only associated with poor RFS and OS in younger patients with TNBC (age < 50 years) (P < .001 and P < .001, respectively). Our results suggest that Le(X) expression is an independent prognostic factor for RFS and OS in younger patients with TNBC.