In patients with iron deficiency anaemia (IDA) it has been suggested that the shortened erythrocyte lifespan may be in part due to decreased erythrocyte deformability. In order to know whether erythrocyte deformability is decreased in IDA patients, we have determined the erythrocyte Elongation Index (EI) by means of ektacytometric techniques (Rheodyn SSD, Myrenne Gmbh, Germany), in 50 IDA patients and 100 well age and sex matched healthy controls. At the three shear stresses tested, 12, 30 and 60 Pa, IDA patients show statistically lower EI than controls (37.4+/-6.7 vs 48.6+/-2.9; 45.0+/-6.0 vs 54.5+/-2.8; 48.7+/-5.8 vs 57.0+/-2.9 mPa.s, respectively; p<0.001). A statistically significant correlation was found between EI at 12, 30, and 60 Pa and the hematimetric indices (MCV, MCH and MCHC), suggesting that the alteration in surface/volume ratio (shape) which characterizes this kind of microcytic hypocromic anaemia, accounts in part for the decreased EI. Rheodyn SSD, as an ektacytometric technique, is very sensitive to alterations in red blood cell geometry, for what seems to be a useful tool for detecting diminished erythrocyte deformability in IDA patients.