Acute lymphoblastic leukaemia (ALL) is the most common cause from death of disease in children. Whilst cure rates over the last 30 years have drastically improved, the children that do go on and relapse have a very poor prognosis. Additionally, the ones that do survive can have significant long term side effects from existing treatments. Understanding the molecular mechanisms of the relationship between leukaemia and its microenvironment is essential for the identification of novel targets for treatment and/or the manipulation of existing treatments. The role that vascular endothelial growth factor (VEGF), an integral component of both neovascularisation and normal haematopoiesis, plays in the progression and invasiveness of solid tumours is well established. However, its function in haematological malignancies has been a more recent and thus less considered observation. Human leukaemia cells secrete VEGF, which may act in a paracrine manner with the bone marrow microenvironment to promote the survival and proliferation of leukaemia cells. In addition to VEGF being produced by leukaemias, it also increases vascularity in the bone marrow and lymph nodes of patients. Our previous work has established a panel of 10 childhood acute lymphoblastic leukaemia xenografts from patient biopsies in NOD/SCID mice. Several of these secrete VEGF, and express the FMS-like tyrosine kinase-3 (FLT-3). FLT 3, a receptor tyrosine kinase (RTK), and its ligand, play an essential role in regulating normal haematopoiesis. This thesis builds on the previous work by examining the relationship between VEGF and FLT 3, two widely, yet independently studied molecules in leukaemia, with the aberrant expression of either having adverse outcomes for patients. The results show that the high expression and activation of FLT 3, significantly increases the secretion VEGF. To assess whether VEGF secretion is triggered by FLT-3 signalling, we measured VEGF in the absence and presence of a class III receptor tyrosine kinase (RTK) inhibitor (SU11657), humanised anti-FLT 3 blocking antibodies as well as decreasing the receptors with siRNA. All of these manipulations were able to decrease the secretion of VEGF in leukaemia cells. To further investigate this relationship, we examined the phosphorylation status of FLT-3 and the downstream signalling pathway. Our results indicate that FLT 3 signalling may be an important factor in the induction of VEGF secretion in a sub-type of leukaemia cells and in turn, VEGF secretion can be attenuated by an FLT-3 specific inhibitor. Two separate microarray studies were also used to assess simultaneous gene expressions between the leukaemia and bone marrow microenvironment, and to examine the effects of FL on ALL xenograft cells. The results of the microarray studies confirm the previously observed results regarding the manipulation of the microenvironment by the leukaemic cells. Inhibition of the FLT-3/VEGF pathway may disrupt paracrine signalling between leukaemia cells and the bone marrow microenvironment, and future studies into how this disruption may influence leukaemia cell responses to conventional chemotherapy are warranted.