Perineural dexamethasone has a ceiling dose of 4 mg for prolongation of analgesia duration after injection of long-acting local anaesthetic for peripheral nerve block, but evidence for doses < 4 mg is lacking. This randomised controlled triple-blind trial tested the hypothesis that increasing doses of perineural dexamethasone between 1 mg and 4 mg would prolong the duration of analgesia in a dose-dependent manner. Eighty ASA physical status 1-2 patients scheduled for shoulder arthroscopy under general anaesthesia with ultrasound-guided interscalene brachial plexus block were randomly allocated to receive saline (control), dexamethasone 1 mg, 2 mg, 3 mg and 4 mg, together with 20 ml ropivacaine 0.5%. Postoperative analgesia consisted of paracetamol, diclofenac and oxycodone on request, using a pre-defined protocol. The primary outcome was the duration of analgesia, defined as the time between the block procedure and the first analgesic request. Secondary outcomes included rest and dynamic pain scores, and analgesic consumption at 2 h, 24 h and 48 h postoperatively. An analysis of the dose-response relationship was performed using multiple comparison procedure-modelling. The median (IQR [range]) duration of analgesia was significantly prolonged in a dose-dependent manner: control 685 (590-860 [453-1272]) min; 1 mg 835 (740-1110 [450-1375]) min; 2 mg 904 (710-1130 [525-1365]) min; 3 mg 965 (875-1025 [730-1390]) min; 4 mg 1023 (838-1239 [518-1500]) min (p = 0.03). There were no significant differences between the secondary outcomes. Perineural administration of dexamethasone with doses between 1 mg and 4 mg, combined with ropivacaine for interscalene brachial plexus block, prolongs duration of analgesia in a dose-dependent manner.