We describe a 31-year-old man in whom a paresis and sensory defect of the left arm developed after amputation of the index finger. The operation was performed in a bloodless field, using a pneumatic tourniquet. The sensory defect resolved in two months and the paresis in five and a half months. We consider that direct pressure produced by the tourniquet caused the nerve lesion. It is probable that the tourniquet was inflated to a pressure of 500 millimetres of mercury instead of the intended 250 millimetres of mercury because of a faulty gauge. In order to avoid this rare complication, it is advisable to check the tourniquet gauge each time before use.