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Objectives Validation of the measurement of erythrocyte deformability as a useful prognostic, rheological biomarker for patients with sickle cell disease (SCD). Methods The degree of reduced deformability was based on the value of the maximum elongation index (EImax) of the deformability curve of an osmotic gradient ektacytometer. The performance of this technique was analytically and clinically validated by analysing 200 normal subjects and 100 patients with well‐documented thalassemia's and Hb variants in relation to their clinical condition. Results In this study, we show that EImax is a reproducible parameter with a small inter‐individual coefficient of (Biological) variation (CV)=1.6% and a small intra‐individual CV=3.5%. We demonstrate that loss of deformability correlates with the clinical condition and the various mutations underlying sickle cell disease and thalassemia. For SCD patients, a strongly reduced EImax with a cut‐off =0.360 is a signal for future vaso‐occlusive (VOC) events requiring hospitalisation with a specificity=85%, sensitivity=80%, PPV=81% and NPV=84% based on a ROC curve (AUC=0.89). Conclusion This study validated the clinical utility of EImax as a prognostic marker for future clinical problems in individual high‐risk SCD patients. In addition, EImax may help to achieve an adequate personal transfusion policy for an optimal blood flow in anaemic patients with SCD.