, all patients' files operated in the surgical unit of the Central Hospital of Yaounde and who underwent an US exploration in the postoperative period through the acquired bone defect or a fracture of the skull were reviewed consecutively. US was performed in all patients operated for posterior fossa tumour to rule out hydrocephalus and in other patients when clinical worsening was observed. US findings were compared with CT scan results and peroperative findings. The US apparatus used was a Siemens Sonograph. Three types of probe were available: (1) a 7-cm diameter convex-shaped probe and frequency 3.5-6 MHz (for near-field lesions), (2) a 2.5-cm diameter probe and frequency 7-12 MHz (for far-field lesions), and (3) a 5-cm diameter probe and frequency 7-12 MHz (for far-field lesions).