Postoperative myelography with water-soluble contrast media was performed in 36 children with a diagnosis of posterior cranial fossa tumour. The myelograms were normal in 15. In 5 an intramedullary tumour was present and 3 of these had in addition subarachnoid changes as evidence of tumour spread. The remaining 16 patient had subarachnoid changes of a different character, mainly located in the posterior thoracic region and similar to those seen after subarachnoid haemorrhage. It is suggested that they represent adhesions caused by blood from the operation. The blood is assumed to be distributed by the large cerebrospinal fluid pulsations to the cervical and thoracic regions. It is important to recognise and differentiate subarachnoid changes due to tumour and to postoperative adhesions to avoid unnecessary radiotherapy to the spinal cord.