A technique is described for the irradiation of the whole brain and spinal cord in the treatment of medulloblastoma and ependymoblastoma. The necessity for shielding part of the posterior X-ray field for a proportion of the treatments, as in earlier irradiation schemes, has been eliminated by modifying the field using a specially designed wedged copper filter. The filter also enables the isodoses to follow approximately the contour of the spinal cord, and to bring the isodoses in the head into line with those arising from an angled cobalt field applied anteriorly through the forehead thus producing uniform dosage throughout the brain. Two such filters, of adjustable length, which can together accommodate a wide range of patient sizes are described in detail. Isodoses for the filters and for a typical patient are included and a method for quickly providing isodoses for any patient is outlined. A special “setting-up” jig for the posterior field facilitates quick and accurately reproducible positioning of the patient and provides immobilisation during irradiation. Clinical reports of the seven patients treated so far are included and, although the follow-up is too short to produce survival figures, only one of the seven has died, this patient being a case of ependymoma.