Objective: To describe a case of acute lateral sinus insufficiency secondary to acute mastoiditis and discuss how lateral sinus thrombosis (LST) is produced.Methodology: Case report and discussion focusing on the anatomical features and the pathophysiology of LST.Result: A 4-year-old boy developed headaches and vomiting after acute otitis media.CT showed a soft tissue density in the right mastoid cavity and the sigmoid sinus narrowed medially by the gas and low density mass.On the opposite side, CT also revealed an unclear bony sinus plate, which was influenced by the well-developed mastoid air cells.We diagnosed the patient with suspected LST and consequently performed simple mastoidectomy.A large amount of bloody pus was found in the mastoid cavity and many clots were observed under the bony sinus.Sigmoid sinus blood flow was completely restored after removing the clots, and the patients symptoms subsequently disappeared.Based on surgical findings, our case was considered to be at a stage prior to LST.