Background The heterogeneity of the pathology of stroke has been a major difficulty in assessing new treatments for acute stroke, and contributes to the complexity of stroke medicine. Some underlying mechanisms are poorly understood, such as small vessel (lacunar) disease. New technology such as advanced brain imaging has transformed our knowledge of large vessel disease and we suggest that other new technology such as detailed analysis of retinal vessels may provide new insights into the pathology of small vessel (lacunar) stroke disease. We hypothesise that retinal microvascular signs differ by pathological stroke subtype, and we plan to test this hypothesis in over 1000 acute stroke patients.Summary Eligible patients undergo a standardised neurological assessment followed by digital retinal photography. At a consensus meeting, an anatomical and aetiological classification is determined. Retinal photographs will be assessed qualitatively (e.g. retinal emboli, arteriovenous nicking) and quantitatively (arteriolar/venule ratio). Six-month vascular event rates together with disability and vital status are collected.Conclusion Retinal vessel appearances may provide a 'window' to the brain and help determine the important underlying pathophysiological mechanisms of small vessel disease stroke.