Objective: Hyperglycemia contributes to various health problems including infectious disease and cancer development. Treatment of lymphoma involve high-dose of corticosteroids and therefore often increase the risk of hyperglycemia. This study was aimed to evaluate whether hyperglycemia during chemotherapy influences the prognosis of patients with non-Hodgkin lymphoma. Methods: We retrospectively reviewed the medical records of all patients with non-Hodgkin lymphoma who were treated with a type of chemotherapy containing a corticosteroid at Chungbuk National University Hospital from January of 2014 to December of 2017. Severe hyperglycemia was defined as the clinical terminology criteria for adverse events categories greater than or equal to 3 (≥ 3; fasting glucose > 250 mg/dL). Results: A total of ninety-seven patients were reviewed. Median age was 65.7 years, and 74.2% were diagnosed with diffuse large-B cell lymphoma. Seventeen patients (17.5%) had type 2 diabetes mellitus (T2DM). During chemotherapy, 22 patients (22.6%) had grade 3 or higher hyperglycemia. The risk factors for hyperglycemia were obesity and underlying T2DM. There was no relationship between hyperglycemia and delayed treatment, reduced doses of chemotherapeutic agents, or the incidence of infectious complications. However, in a multivariate analysis of overall survival, hyperglycemia [hazard ratio (HR) 2.9, P = 0.004] was a significant risk factor for mortality, similar to high international prognostic index (IPI) score (HR 2.2, P = 0.019). On the other hand, T2DM itself was not directly related to overall survival. The one-year survival rate of patients with hyperglycemia tended to be lower than those without hyperglycemia (37.5% vs 67.4%, P = 0.089). Conclusion: The occurrence of severe hyperglycemia during chemotherapy is associated with low survival rates in lymphoma patients. In patients with diabetes, blood glucose levels should be carefully controlled during lymphoma treatment.