Membranous tissue under the flexor carpi ulnaris (FCU), hypertrophy and fascia of the FCU were examined in 90 patients with cubital tunnel syndrome; it (submuscular membrane) was found in all patients, excluding five whose data were incomplete. Impressions on the nerve under the FCU were noted in 75 patients. In 43 patients, the submuscular membrane was judged to be the cause of neural compression. These 75 patients included 60 and 15 cases whose fascia of the FCU were judged to the thick and thin, respectively. Among the 58 with a thick submuscular membrane, 56 patients had an impression under the membrane. Fifteen patients had a flat impression on the nerve in the proximal compartment of the FCU, which was formed by ligamentous and bony floor and thick fascia of the FCU. It is believed that increased pressure in the compartment is the cause of a flat impression. Slight compression of the nerve under the FCU with compressions in proximal and distal part of the nerve can cause symptoms of the nerve compression by multiple crush effect.