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BACKGROUND Plain 2-chloroprocaine , a rapid acting amino-ester-type local anaesthetic, is used in short ambulatory procedures under spinal anaesthesia. The ED95 of 2-chloroprocaine for lower limb surgery has not yet been defined. Moreover, patients’ body height could influence the effects of the administered dose. OBJECTIVES The aim of this study was to determine the ED95 of plain 2-chloroprocaine based on the height of patients undergoing ambulatory knee arthroscopy under spinal anaesthesia using the continual reassessment statistical method (CRM). DESIGN Prospective, dose-response, double blind, controlled, bi-center, clinical trial. SETTING One university teaching hospital in Brussels and one general teaching hospital, Braine l’Alleud, Belgium. Université libre de Bruxelles, PATIENTS ASA I-III (120 patients) scheduled for day-case knee arthroscopy under spinal anaesthesia INTERVENTIONS Patients were divided into three groups of 40, each comprising 10 cohorts of 4 patients, according to their respective heights (Group 1: 150–165 cm, Group 2: 166–180 cm, Group 3: 181–195 cm). Each patient enrolled into one of the 3 groups received an intrathecal dose of plain 2-chloroprocaine determined by the CRM. The starting doses for the first cohorts of four were 40 mg in group 1, 45 mg in group 2 and 50 mg in group 3. The doses for subsequent cohorts were determined by CRM of outcomes in all patients in the previous competed cohorts. Anaesthesia was considered successful when there was a lack of pinprick or cold sensation up to T12, the visual analogue scale score for tourniquet pain was < 2 and no pain during surgery. MAIN OUTCOMES The primary outcome was the success or failure of the block. Side effects were also recorded. RESULTS ED95 of 2-chloroprocaine is 40 mg in group 1, 35 mg in group 2, 45 mg in group 3. Groups were different in terms of height and gender (P < 0.05) and similar in terms of body mass index and age (P > 0.05). CONCLUSION The ED95 of intrathecal 2-chloroprocaine for patients undergoing ambulatory knee arthroscopy is between 35 and 45 mg. Height could be a factor involved in the calculation, and patient sex could partly explain the nonlinear relation between height and dose. TRIAL REGISTRY NUMBER Clinicaltrials.gov NCT03882489.