Although vascular density and vascular endothelial growth factor (VEGF) levels are increased in acute myeloid leukaemia (AML), anti-VEGF therapy has been ineffective in clinical trials. Mainly studied in vitro, the crosstalk between leukaemic cells and endothelial cells (ECs) remains poorly understood. Using two-photon microscopy, a new study has investigated the bone marrow vasculature in AML in vivo. Engraftment of patient-derived AML cells into mice increased vascular permeability compared with control mice engrafted with normal human haematopoietic stem and/or progenitor cells, an effect that persisted after treatment with the AML chemotherapy cytarabine. Interestingly, the NAPDH oxidase Nox4 was the most upregulated common gene in bone marrow-derived ECs of AML-engrafted mice, and was associated with high production of reactive oxygen species in the bone marrow as well as induction of nitric oxide (NO) synthase 3 (NOS3) in ECs. Accordingly, levels of NO, an inducer of vascular permeability, in the bone marrow of AML-engrafted mice were increased compared with control mice. Pharmacological NOS inhibition in combination with cytarabine-reduced AML progression and vascular permeability in the mice, providing evidence for a potential clinical benefit of targeting NOS in the bone marrow vascular niche in AML.