Sixty seven infantile patients developing postoperative phlogistic complications as a consequence of posterior fossa surgery (i.e.: neoplasms, arachnoid cysts, A-V malformations) are herein studied. They have been subdivided in 2 groups in accordance to the different preoperative diagnostic procedures they underwent. In the first series 41 cases of posterior fossa anomalies have been diagnosed by means of air contrast ventriculography with or without cerebral angiography; in the other one the 26 patients were submitted to C.A.T. with or without angiography. In the first group the incidence rate of phlogistic complications was 31.7%, in the second one 3.8%. According to the authors this considerable difference is due to the diagnostic procedures employed. Contraindications to air contrast ventriculography and advantages of CT scan are widely discussed and emphasized.