We studied median nerve involvement in a group of asymptomatic handworkers at risk for carpal tunnel syndrome, and we evaluated damage to thin and thick nerve fibres in the distribution area of the median nerve. Considering floor cleaners as workers at high risk of developing cumulative traumatic disorders in the wrist, we included 42 cleaners and 41 controls. We assessed nerve conduction studies, vibration threshold, and temperature and pain thresholds of the median nerve. The cleaners had significantly impaired motor nerve conduction velocity (p = 0.006), longer sensory distal latency (p = 0.01), lower sensory amplitude (p = 0.0005), and increased difference in heat and cold threshold of the median nerve (p = 0.0002). Increased temperature threshold was associated with prolonged sensory distal latency of the median nerve in the cleaners. In conclusion, impaired neurophysiological variables in the median nerve in floor cleaners compared with controls confirm the hypothesis that those workers are at risk of developing median nerve dysfunction. Sensory nerves seem to be more susceptible to injury than motor branches.