Objective To discuss the curative effect of expanding ulnar nerve groove and interfascicular neurolysis under microscope in treating severe cubital tunnel syndrome (Cub Ts), and to compare with that of the forward moving of ulnar nerve and interfascicular neurolysis under microscope to find out the best way to treat severe Cub Ts. Methods From December 2002 to January 2007, 22 severe Cub Ts cases were treated with expansion of ulnar nerve groove and interfascicular neurolysis under microscope (treatment group), and other 22 cases were treated with forward moving of ulnar nerve and interfascicular neurolysis under microscope (control group). In treatment group, there were 17 males and 5 females, aged 21-66 years (mean 43.8 years). Pathogenic causes were elbow arthritis in 17 cases, ulnar nerve dislocation in 3 cases and elbow ectroption in 2 cases. The locations were left elbow in 8 cases and right elbow in 14 cases. Thecourse of disease was 6-69 months. In control group, there were 18 males and 4 females, aged 20-64 years (mean 42.1 years). Pathogenic causes were elbow in arthritis 16 cases, ulnar nerve dislocation in 3 cases, elbow ectroption in 1 case and narrowing and shallowing of ulnar nerve groove caused by abnormal heal ing of medial condyle fracture in 1 case. The locations were left elbow in 7 cases and right elbow in 15 cases. The course of disease was 5-67 months. Results For all patients of both groups, the wound healed by first intention, and all were followed up for 12-45 months. In treatment group, the numbness in l ittle finger was obviously rel ieved, or disappeared in 22 cases 1 day after operation. In control group, the numbness in l ittle finger was obviously rel ieved or disappeared in 22 cases 3-5 days after operation. EMG showed that conduction speed of ulnar nerve was normal. Evaluated by upper l imbs function standard of China Medical Association, Surgery Association and Lascar grades, the results were excellent in 21 cases and good in 1 case in treatment group; whilet excellent in 19 cases, good in 2 cases and fair in 1 case in control group. There was significant difference between treatment group and control group (P lt; 0.01). Conclusion Either expansion of ulnar nerve groove and interfascicular neurolysis or forward moving of ulnar nerve and interfascicular neurolysis is an effective method to treat severe Cub Ts, but the former is better than the latter.