['Chronic Cerebrospinal Venous Insufficiency (CCSVI) has been associated with multiple sclerosis (MS) with a risk ranging from as high as two-hundred-fold to a protective effect. However, not all studies were blinded, and the efficacy of blinding was never assessed.', 'To evaluate the association of CCSVI with MS in a cross-sectional blinded study and look for any association of CCSVI with the severity of MS.', "The Echo-color Doppler examination was carried out in accordance with Zamboni's five criteria in 68 consecutive MS patients and 68 healthy controls, matched by gender and age (±5 years). Four experienced neurosonologists, blinded to the status of cases and controls, performed the study and were then asked to guess the status (case or control) of each participant. The number of positive CCSVI criteria was similar in the two groups. CCSVI, defined as the presence of two or more criteria, was detected in 21 cases (30.9%) and 23 controls (33.8%), with an OR of 0.9 (95%CL\u200a=\u200a0.4-1.8, p\u200a=\u200a0.71). The prevalence of CCSVI was related to age in cases (OR increasing from 0.2 to 1.4), but not in controls. CCSVI positive (N\u200a=\u200a21) and negative (N\u200a=\u200a47) MS patients were similar in clinical type, age at disease onset, disability, and fatigue. Disease duration was longer (16.5±9.8 years) in CCSVI positive than negative patients (11.5±7.4; p\u200a=\u200a0.04). The operators correctly guessed 34/68 cases (50%) and 45/68 controls (66%) (p\u200a=\u200a0.06), indicating a different success of blinding."] Is chronic cerebrospinal venous insufficiency associated with multiple sclerosis and its severity : a blind-verified study?