PURPOSE: This study assessed the determinants of 6-month survival of critically ill patients with an active hematologic malignancy (HM). METHODS: All patients with an active HM defined by either receiving ongoing or due to receive antineoplastic therapy, admitted to 2 tertiary intensive care units between 2010 and 2015, were included in this retrospective cohort study. RESULTS: Of the 273 patients included in the study (median age, 63[interquartile range, 54-71] years; 40.7% female), 116 (42.5%; 95% confidence interval, 36.8-48.4) died in hospital. The 6-month mortality was 56.4% (95% confidence interval, 50.5-62.2). Mechanical ventilation, intensive care unit admission source, and the type of active HM were significantly associated with hospital mortality and 6-month survival, after adjusting for severity of acute illness. The type of active HM was the most important prognostic factor, with over a 10-fold difference in 6-month survival between HM with the best and worst prognosis. In addition, recent hematopoietic stem cell transplant (<30 days) was associated with a better 6-month survival. CONCLUSION: Differences in 6-month survival between critically ill patients with different types of active HM were substantial. Recent hematopoietic stem cell transplant, severity of illness, and use of mechanical ventilation were additional important determinants of 6-month survival in patients with an active HM.