548 Background: Cytotoxic chemotherapy can cure patients with advanced germ cell tumors. Nevertheless, cancer treatment may induce cellular senescence and accelerate molecular aging. The aging process implies an increase of cells expressing p16INK4a and changes in lymphocyte subpopulations. Our aim was to study the potential induction of premature immunosenescence in testicular cancer patients exposed to chemotherapy. Methods: Case-control study of testicular cancer survivors (TCS) treated with chemotherapy (≥3 BEP cycles, disease-free ≥ 3 months) matched by age with healthy controls. Peripheral blood mononuclear cells were isolated, and lymphocyte subpopulations were analyzed by flow cytometry. p16INK4a expression was measured using real-time polymerase chain reaction, and the 2^-ΔΔCt method was used for the p16INK4a expression analysis. Groups were compared with Mann-Whitney U test. Results are expressed as median (IQR) unless otherwise indicated. Results: We included 15 cases and 15 controls. Mean age 32.5 yo (24-54), mean time since last chemo 49.5 months (3-192). TCS had a lower percentage of CD3+ (62 (57-68) vs 72 (65-82) p = 0.026) and CD4+ (35 (27-41) vs 42 (38-53) p = 0.013) cells in total lymphocytes. TCS also had a lower % of naïve CD4+ (32 (15-44) vs 39 (32-55) p = 0.041), CD4+CD28+ (90 (85-97) vs 98 (95-99) p = 0.029) and CD8+ naïve (15 (8-26) vs 27 (22-42) p = 0.023) cells. TCS had an increased % of memory CD4+ (18 (13-26) vs 9 (6-11) p = 0.001) and memory CD19+ B cells (76 (62-88) vs 70 (64-79) p = 0.004). The relative expression of p16INK4a in CD3+ cells was higher in TCS compared to controls (1.29±0.36 vs 0.85±0.54; p = 0.035). Conclusions: TCS showed an increase in the expression of the aging biomarker p16INK4a and a lymphocyte phenotype associated with immunosenescence; characterized by a decrease in naïve cells, and concomitant increment of memory and CD28- cells. This phenomenon is believed to contribute to the development of an immune risk profile in the elderly, associated with an increased rate of infections and a diminished effect of vaccines. This is the first time this finding is reported in TCS. Further studies are warranted to define the clinical implications of this alteration.