Background: The main objective was to determine whether the administration of chemotherapy (CT) during the month before intensive care unit (ICU) admission of medical patients with cancer influences the survival rate. The design was a single-institution observational cohort study in an ICU of a tertiary university hospital. Methods: Our cohort included 248 oncology patients admitted to the ICU from 2005 to 2014 due to nonsurgical problems. Seventy-six (30.6%) patients had received CT in the month before admission (CT group) and 172 did not receive CT (control group). The main outcome measures were ICU, hospital, 30-day, 90-day, and 1-year mortalities. We performed survival analysis using the Kaplan-Meier estimator, comparing both groups using the log-rank test, and multivariate analysis using Cox regression adjusted for gender, age, maximum Sequential Organ Failure Assessment (SOFA), and delta maximum SOFA to calculate the hazard ratios (HRs) and their respective 95% confidence intervals. This association was also evaluated by a graphic representation of survival. Results: The CT group presented an ICU mortality rate of 27.6% versus 25.5% in the control group. The multivariate analysis adjusted for age, sex, and delta maximum SOFA showed significant differences between the groups (HR: 2.12; P = .009). The hospital mortality rate was 55.3% in the CT group compared to 45.4% in the control group (adjusted HR: 1.81; P = .003). At 30 days, the mortality rate was 56.6% in the CT group compared to 46.5% in the control group (adjusted HR: 1.69; P = .008). Mortality at 90 days was 65.8% in the CT group versus 59.9% in the control group (adjusted HR: 1.47; P = .03). One-year mortality was also higher in the CT group (79% vs 72.7%, adjusted HR: 1.44; P = .02). Conclusion: The administration of CT in the month before ICU admission in patients with cancer was associated with higher mortality in the ICU, in the hospital, and 30 and 90 days after admission when adjusted for the increase in organ failure measured by delta maximum SOFA. We provide useful new information for decision-making about ICU management of patients with cancer.