Twelve median and 12 ulnar nerves were obtained at routine autopsies from patients without known disease of the peripheral nervous system. Enlargement of cross-sectional area due to an increase in connective tissue elements was commonly present in the ulnar nerve at the elbow and in the median nerve under the flexor retinaculum. Renaut bodies were also prominent at these two sites. The connective tissue changes did not appear to be related to the presence or absence of nerve fibre damage. When nerve fibres were teased apart and examined individually, localised changes were found at the elbow in 5 ulnar nerves and under the flexor retinaculum in 5 median nerves. The changes were mild and transverse sections at the same levels showed few abnormalities. However, the changes were similar in character to those described previously in experimental animals with entrapment syndromes. They are therefore considered to be valid evidence of sub-clinical entrapment in apparently unaffected human subjects.