The effect of the urinary calcium concentration (CaU) on erythrocyte morphology was studied by incubating erythrocytes in urine with prefixed CaUs of 5, 10, 20 and 40 mmol/l by addition of CaCl<sub>2</sub>. The same experiment was carried out on erythrocytes preincubated with levo-verapamil (1-V) at 10,100 and 200 μmol/l. Phase contrast microscopy observations were performed at 0, 30, 60, 120, and 240 min by 2 experienced investigators. At 0 min the erythrocytes showed a clear extraglomerular pattern. At 60 min marked morphological and volumetric alterations were evident when the CaU was > 10 mmol/l. On the contrary, no change was found when red cells were treated with > 100 μmol/l 1-V, independent of the CaU. Dysmorphic erythrocyturia has been related to transglomerular passage even if it was sporadically observed in hypercalciuric or lithiasic patients. This work suggests a role for a high CaU in causing the formation of microcytic and warped erythrocytes. In our opinion, in hypercalciuric urine the appearance of dysmorphic or mixed hematuria does not necessarily indicate transglomerular passage.