Background:The consequences of infectious toxicity of hypomethylating agents (HMAs) on overall survival (OS) of patients diagnosed with high-risk myeloid neoplasms have not been thoroughly investigated.Objectives:We aimed to evaluate whether infectious events (IEs) negatively influenced the results of HMA treatment in a real-world setting.Design:Observational study.Methods:We obtained data from 412 non-selected consecutive patients from 23 Spanish hospitals who were diagnosed with high-risk myelodysplastic syndrome, chronic myelomonocytic leukemia, or acute myeloid leukemia and were treated with HMA. HMAs received after chemotherapy or stem cell transplant were excluded. All IEs were recorded. Outcomes included OS, modifications to the pre-planned treatment, incidence and characteristics of IEs, hospitalization, red blood cell transfusions, and factors associated with infection.Results:The rate of infection was 1.2 per patient/year. Next-cycle delay (p = 0.001) and hospitalizations (p = 0.001) were significantly influenced by IEs. Transfusion requirements during each cycle were significantly higher after infection compared with cycles without infection (coefficient = 1.55 [95% confidence interval (CI) = 1.26–1.84], p < 0.001). The median number of cycles was lower in patients experiencing any infection during the first four cycles (5 [3–8] versu 8 [5–16], p < 0.001). In the multivariable analysis, factors associated with lower OS were having any infection during the first four cycles (hazard ratio (HR) = 1.43 [95% CI = 1.09–1.88], p = 0.01), bone marrow blasts ⩾30% (HR = 2.13 [95% CI = 1.14–3.96], p = 0.01), adverse cytogenetics (HR = 1.70 [95% CI = 1.30–2.24], p < 0.001), and platelet count 20% (HR = 1.57 [95% CI = 1.19–2.01], p < 0.001) and adverse cytogenetics (HR = 1.7 [95% CI = 1.35–2.14], p < 0.001) were associated with infection, whereas hemoglobin >9 g/dl (HR = 0.65 [95% CI = 0.51–0.82], p < 0.001) and higher platelet count (HR = 0.997 [95% CI = 0.996–0.998], p = 0.016) protected from it.Conclusion:HMA infectious toxicity worsens OS, hinders the adherence to antineoplastic treatment and results in significant morbidity. Preventive strategies are fundamental in vulnerable patients.