We evaluated the clinical usefulness of colonoscopic insertion of a decompression tube (decompression method) for the treatment of ileus associated with left-sided colorectal cancer. Decompression method was done in 48 patients with colorectal cancer (38 primary cancer, 10 metastatic cancer). A decompression tube was successfully inserted in all but 10 patients who had primary cancer with severe strictures. The overall insertion rate was 79%. Decompression method improved obstructive symptoms and decreased intestinal gas as evaluated on plain X-ray films of the abdomen. Emergency operation was unnecessary in 96% of the patients with primary cancers, in whom the decompression tube was successfully inserted. We conclude that decompression method can improve abdominal symptoms caused by obstructive colorectal cancer and reduce the need for emergency operation.