Brainstem acoustic evoked potentials (BAEPs) were recorded in 16 children with posterior fossa tumours. The results were compared with the clinical course, CT scan findings and the postmortem pathological findings. Eight children had midline medulloblastoma, four children had lateral astrocytoma (two of them cerebellar, one of them pontocerebellar, and one of them craniospinal) and four children had brainstem glioma. Different BAEP pattern could be detected according to the tumour's location and histological nature: 1. In midline medulloblastoma: bilateral, symmetrical, or slightly asymmetrical I-V. IPL prolongation was the common abnormality. Beside that, some of the patients showed III-V., or I-III. IPL prolongation, or V. depression. 2. In lateral astrocytoma: asymmetrical BAEP abnormalities were seen: unilateral V. depression, and/or I-V. IPL prolongation. 3. In brainstem glioma: severely distorted waveform could be observed, with depression and gradual disappearance of components following the wave III. Based on these results BAEP measurement seems to be useful in the differential diagnosis of posterior fossa tumours of childhood, and it can be necessary even beside the CT scan. It can be useful in the early diagnosis, because the BAEP positivity can precede the CT scan positivity, in such a case repeated CT scan required. Finally BAEP measurement proved to be effective in the follow-up of posterior fossa tumours: it can document the tumour's chemotherapeutic regression, or the progression. It is a simple, non-invasive, and cheap method.