OBJECTIVE: To present a case of pediatric cholesteatoma that invaded the petrous apex (PA) and discuss the usefulness of preoperative three-dimensional (3D) surgical simulation on a personal computer (PC) and patient-specific 3D printed model-assisted surgery. PATIENT: A 5-year-old boy with congenital cholesteatoma underwent a planned two-stage canal wall up mastoidectomy. The cholesteatoma had invaded the PA from a small space anterior to the superior semicircular canal (SSCC). During the removal of this lesion in the first surgery, the tip of a 1-mm round knife broke off and fell into the PA. The surgeon could not remove it, as it was thought that opening the space might damage the SSCC and the facial nerve (FN). INTERVENTION: Before the second surgery, a preoperative 3D surgical simulation on a PC was performed, and an approach to the PA via the triangle surrounded by the SSCC, FN, and middle cranial fossa, namely, the supracochlear approach, was discovered. A patient-specific 3D-printed model, which had been drilled to make each surface of the triangle including the SSCC, FN, and middle cranial fossa visible in the PC simulation surgery, was then created and a 3D-printed model-assisted surgery was planned. RESULTS: By placing the sterilized patient-specific 3D model close to the surgical field, the cholesteatoma and iatrogenic foreign body could be successfully removed from the PA without damaging the important surrounding structures. CONCLUSIONS: Preoperative 3D surgical simulations and intraoperative patient-specific 3D-printed model-assisted surgeries are new, powerful tools that aid in performing challenging surgeries on temporal bones.