e16037 Background: Cytotoxic (Ctx) chemotherapy can cure patients with advanced testicular cancer. Notwithstanding, Ctx agents could promote molecular aging and induce cellular senescence in vivo. This phenomenon has been studied in breast cancer patients exposed to adjuvant chemotherapy, however, the potential induction of premature immunosenescence in testicular cancer survivors (TCS) exposed to chemotherapy remains unknown. Methods: Case-control study. Cases were 18-55 yo TCS, treated with chemotherapy (≥3 BEP cycles), disease-free for ≥3 m. Controls were healthy subjects matched by sex and age ± 1 y. Selected subpopulations of isolated peripheral blood mononuclear cells (PBMC) depicted in Table, were analyzed flow cytometry. Statistics: Adjusted counts or relative percentage for each population were compared using student-t test or Mann-Whitney U test as appropriate. Results: We included 15 TCS and 15 controls. Mean age was 32.6 yo (24-54 y). Mean time since last chemotherapy was 53.13 months (3-192 m). All 15 patients were treated with at least 3 BEP cycles ± TIP/VIP cycles. There were no differences in total leucocyte (5128 ± 1917 vs 5850 ± 2141, p=0.36), and lymphocyte (1539 ± 592 vs 1889 ± 825, p=0.4) counts among cases and controls. Cases have a diminished percentage of CD3+ (63 ± 10 vs 71 ± 11, p=0.05) and CD4+ cells (36±8 vs 43 ± 9, p=0.03). There were no differences in pct of CD8+ or CD19+. However, there was an increase in late differentiated CD8+/CD57+ cells (31 ± 14% vs 20 ± 11%, p=0.02); and a decrease in CD4+/CD28+ cells (90 ± 9% vs 97 ± 4%, p=0.02) in cases. We observed paradoxical changes in B cell subpopulations with an increment in naïve (79 ±10 vs 67% ± 14% p=0.02) and a decrease in memory (18 ± 9 vs 31 ± 13%, p=0.01) B cells. Conclusions: TCS after chemo develop changes compatible with immunesenescence in T cell (CD3+) compartment, and an increment in naïve B cells. [Table: see text]