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Increases in skin temperature may be used as an early predictor of the success of interscalene brachial plexus block (IBPB), but we lack detailed information on the thermographic response.To investigate and characterise the thermographic response after IBPBs.Prospective observational study.University hospital and private hospital.Twenty-nine male and 17 female patients scheduled for ambulatory shoulder surgery. Exclusion criteria were age less than 18 years, body weight more than 120 kg and any coagulation abnormality.Infrared thermographic imaging of the hand before and at 1 min intervals for 30 min after an ultrasound-guided IBPB with 20 ml ropivacaine 7.5 mg ml−1. Cooling of both hands was performed to standardise measurements.Thermographic changes in skin temperature on the dorsum of the hand.Forty-four blocks were successful and two were failures. Four thermographic patterns were observed after successful blocks: the increase in skin temperature was restricted to the thumb (n = 5); increase in skin temperature of the thumb and the second digit (n = 11); increase in skin temperature of the thumb, the second and fifth digits (n = 4); and an increase in skin temperature in all parts of the hand (n = 24). All successful blocks demonstrated a significant (P < 0.0001) increase in median (range) of distal skin temperature of the thumb of 6.6°C (0.7 to 17.2) by 30 min, which was already significant (P < 0.0001) by 5 min. By contrast, skin temperature decreased significantly (P < 0.0001) in the hand after failed blocks and in the contra-lateral non-blocked hand by −1.5°C (−6.2 to 4.2).Successful IBPB resulted in four thermographic patterns. Skin temperature always increased on the thumb within 30 min and this increase achieved statistical significance at 5 min after the block.