Although there are several studies dealing with erythrocyte deformability (ED) in obese patients, research on this topic in morbidly obese subjects is scarce. In these studies ED seems to be decreased, although the cause remains unknown.A case-control study in 76 morbid obese subjects (23 women and 53 men, aged 44 ± 13 years) and in 79 normal-weight controls (30 women and 49 men, aged 43 ± 13 years) was undertaken. ED has been determined by ektacytometric techniques in a Rheodyn SSD, by means of the elongation index (EI) at 12, 30 and 60 Pascals, along with anthropometric, lipidic, metabolic and inflammatory parameters.EI was statistically lower in morbidly obese subjects than in controls at all the shear stresses tested (EI12: 47.3 ± 2.14 vs. 47.9 ± 2.07; p = 0.047, EI30: 52.16 ± 2.1 vs. 53.12 ± 1.4; p = 0.007, EI60: 53.9 ± 2.4 vs. 55.2 ± 2.50; p = 0.001) as were anthropometric lipidic and inflammatory parameters (p < 0.001). In the bivariate correlation EI60 correlated negatively with most anthropometric, lipidic and inflammatory parameters. However, in the multivariate analysis, the case-control status was not significantly associated with EI60 and only triglycerides, glucose, hs-CRP and waist circumference were independently associated with EI60, constituting independent predictors of altered ED although, waist circumference, showed the highest statistical significance (p = 0.007).ED is decreased in morbidly obese subjects associated with insulin resistance and inflammation parameters although abdominal obesity seems to be of paramount importance in altering this rheological parameter.