Diabetic retinal neovascularisation is considered to be a consequence of retinal ischaemia caused by capillary occlusion. Capillary occlusion is the result of microvascular thrombi in which erythrocytes, platelets and leucocytes each may play a role. We investigated the role of leucocytes in this process and the subsequent angiogenic response. We studied the serum levels of the soluble leucocyte adhesion molecules soluble E-Selectin, soluble vascular cell adhesion molecule-1 (sVCAM-1) and soluble intercellular adhesion molecule-1 (sICAM-1) in the serum of 93 patients with insulin-dependent diabetes (IDDM) and varying degrees of retinopathy and 47 healthy age and sex matched control subjects. We also measured the ability of serum to stimulate retinal capillary endothelial cell migration using an assay of angiogenesis in vitro. Soluble E-Selectin and sVCAM-1 levels were raised in all patients with IDDM (p < 0.001; p < 0.001) particularly those with retinopathy (p < 0.001; p < 0.001). Soluble E-Selectin levels were highest in the patients with severe non-proliferative diabetic retinopathy (p < 0.001) and sVCAM-1 levels were highest in patients with proliferative diabetic retinopathy (p < 0.01). In contrast soluble ICAM-1 levels were the same in patients and control subjects (p > 0.05). Soluble E-Selectin levels in diabetic patients were correlated with the level of glycated haemoglobin (p < 0.05). Retinal endothelial cell migration-inducing (ECMI) activity was increased in patients with IDDM (p < 0.01) in particular in those with retinopathy (p < 0.01). Furthermore, in vitro ECMI activity could be blocked by antibodies to sVCAM-1 and sE-Selectin. These data point to a functional role for leucocyte adhesion in the microvasculopathy of diabetic retinopathy and may have implications for the induction of retinal angiogenesis.