AIMS--To localise the source of bleeding in the urinary tract in patients presenting with haematuria. METHODS--Urine samples were obtained from 109 patients with symptoms referable to the urinary tract. The sample was examined for the presence of red blood cells by phase contrast microscopy (PCM) and the proportion of dysmorphic and isomorphic red blood cells was determined. If more than 20% of the red blood cells were dysmorphic a glomerular origin for the site of bleeding was suspected; if less than 20% of the red blood cells were isomorphic a non-glomerular origin was suspected. Phase contrast microscopy and clinical findings were correlated. RESULTS--The correct bleeding site was shown in 27 of 30 (90%) patients with glomerulopathy and in all 17 patients with bleeding from the lower urinary tract, indicating that this method of analysis has a sensitivity of 90% and specificity of 100% for detecting the glomerular source of bleeding. CONCLUSIONS--The examination of urine for dysmorphic and isomorphic red blood cells by phase contrast microscopy is strongly recommended in routine clinical practice for the detection of glomerular and non-glomerular lesions. This technique may avoid unnecessary investigations for the diagnosis of the site of bleeding in patients with haematuria.