Erythrocyte morphology by phase contrast microscopic examination (PCM) of the urine is widely employed in distinguishing glomerular from nonglomerular bleeding. The proposed percentages of dysmorphic red cells are significant for glomerular bleeding in the range of 10 to 80% in the literature, because there is no clear cut definition of "dysmorphism." In the present study midstream urine samples of 351 patients with hematuria (greater than 8 erythrocytes/microliters) and of 33 healthy controls were examined. The various dysmorphic red cells were analyzed by PCM according to a detailed hematological classification. Most of the dysmorphic red cells, such as echinocytes, anulocytes, ghost cells, schizocytes, stomatocytes, codocytes and knizocytes, occurred in glomerular or nonglomerular disease as well, and proved to be uncharacteristic for glomerular bleeding. In contrast, a unique red cell deformity, a ringform with vesicle-shaped protrusions (acanthocyte) closely correlated to glomerular disease. In biopsy proven glomerulonephritis acanthocytes comprised 12.4% of all excreted red cells, whereas in nonglomerular diseases or in healthy subjects acanthocytes were seen very rarely (less than 2%) or not at all. Acanthocyturia greater than or equal to 5% (of excreted red cells) was seen in 75 out of 143 patients with proven glomerulonephritis (sensitivity 52%) and in four out of 187 patients with nonglomerular disease (specificity 98%). To improve the diagnostic value of erythrocyte morphology the diagnostic workup should focus on acanthocyturia, which is also indicative in very low erythrocyte counts.