Endocan is a soluble proteoglycan expressed only by vascular endothelium and is also found circulating in the bloodstream. Inflammatory cytokines as well as proangiogenic growth factors increase its expression, and increased serum levels are found in immunocompetent patients with sepsis. We investigated serum endocan levels in patients with untreated acute myeloid leukemia (AML) and AML patients during chemotherapy-induced bone marrow failure. We observed increased levels in 40 AML patients compared with healthy controls, which was also confirmed in a second cohort. The serum levels decreased after intensive chemotherapy and subsequent severe chemotherapy-induced cytopenia, and increased levels were thereafter observed during bone marrow regeneration. However, even for these severely immunocompromized patients, serum endocan levels increased during complicating bacterial infections before a decrease was seen during antibiotic therapy. To conclude, serum endocan is a disease marker in AML, but serum levels are also affected by complicating infections and bone marrow regeneration.