SummaryChronic inflammation is often indicated by a relative increase in the gamma globulin fraction in the serum electrophoresis. In a retrospective study, we analyzed the prevalence of relative hypergammaglobulinemia in 60 patients with chronic myelomonocytic leukemia (CMML), its potential prognostic impact, and potential correlations with laboratory and molecular features. Relative hypergammaglobulinemia (> 20%) was found in 25/60 (42%) patients. The median survival of patients with relative hypergammaglobulinemia was significantly shorter than in patients without hypergammaglobulinemia (10 vs. 24 months, p = 0.018). There was no difference between the groups regarding leukocyte count, hemoglobin value, and platelet count, but a higher prevalence of NRAS mutations and a lower prevalence of ZRSR2 mutations in patients with hypergammaglobulinemia. Our results show that hypergammaglobulinemia is present in a proportion of CMML patients and that this abnormality is associated with poor overall survival. The role of chronic inflammation in the pathophysiology of CMML needs to be further investigated.