Disturbances of the peripheral nervous system, summarised under the term "diabetic polyneuropathy", are a well-known complication of long-term diabetes mellitus. However, there is little information about the fact that these conditions may lead to difficulties in monitoring neuromuscular blockade by peripheral nerve stimulation during general anaesthesia. We report two diabetic patients (40 resp. 62 years of age) in whom a total of four ophthalmological operations were performed under general anaesthesia. In all cases monitoring of neuromuscular transmission was attempted by stimulation of the left ulnar nerve slightly proximal to the wrist, but not even the usual calibration could be obtained in either patient. Checks confirmed that the nerve stimulator was functional and the electrodes--in all cases self-adhesive, pregelled surface electrodes--were placed correctly. In order to evaluate the hypothesis that disturbances of peripheral nerve function might be responsible, the patients were examined neurophysiologically in the postoperative period. Both patients showed severe alterations in peripheral nerve conduction: a sensory response could not be evoked in any of the extremities and the ulnar distal motor latency time was pathologically prolonged (5.4 ms in patient 1 and 4.9 ms in patient 2; normal: 1.4-4.0 ms). Therefore, in accordance with the clinical symptoms, the diagnosis of diabetic polyneuropathy was established. These cases demonstrate that severe disturbances of the peripheral nervous system may render neuromuscular monitoring impossible.