The post-traumatic leptomeningeal cyst, so-called growing fracture, is one of the complication of the skull fracture in childhood especially under the 3 years old, but very rare in the adult. Since Taveras and Ranschoff proposed the mechanism for production of the expanding fracture post-traumatically in 1953, many authers have explained the mechanism clinically and experimentally. Now we proposed a additional mechanism for producing the brain herniation into the expanding fracture as a case of post-traumatic leptomeningeal cyst reported. A 25 years old male patient was admitted to the department of Neurosurgery, Seoul Red Cross Hospital, because of headache, convulsion, and growing the mass with skull defect. The patient has had a history of skull fracture at the age of 2 years old. The simple skull films revealed a scalloped and saucerized margin of skull defect on left parietal bone. On electroencephalogram, we found a moderated abnormalities in the left parietal area. The left carotid angiogram showed a stretching of middle cerebral artery and elevation of left Sylvian point. The small capillary branches of middle cerebral artery has pushed out through the skull defect, which indented brain herniation into the skull defect. No cystic picture was found. We operated this case with the dura repair and grafting, and then cranioplasty with resion. On operative finding, we found that one edge on torn dura has impacted into the opposite edge of fractured skull tightly, which suggested the pulling up the dura and brain into the skull defect by growing rate of the skull bone. By the craniectomy for the bone, impacted dura, was free from the impacted bone and the above continued procedure had completely successful . The post-operative course was very satisfactory. According to the operative finding, the skull fracture with dural tearing are essential factors in producing the post-traumatic leptomeningeal cyst in infancy(usually under the 3 years old). When the one edge of torn dura is impacted into the edge of same side of fractured bone, or was left free, it will produce the simple leptomeningeal cyst during the improportional growing the skull bone and brain. On the other hand, when one edge of torn dura is impacted into the opposite edge of fracture bone, the growing parietal bone pulled up the encircled brain into the skull defect aperture gradually, which produced the brain herniation, a type of post-traumatic leptomeningeal cyst. We reported a case of the post-traumatic leptomeningeal cyst in adult very rarely, and proposed a mechanism for producing a brain herniation into skull defect, a type of post-traumatic leptomeningeal cyst.