A technique is described for uniform light irradiation of the tumour cavity after radical subtotal malignant brain tumour resection in patients whose tumours have been photosensitized by intravenous injection of hematoporphyrin derivative. The cavity is maintained by an inflatable rubber 'balloon' filled with a light-scattering liquid, within which is placed a single-strand optical fibre coupled to an argon/dye laser operating at 630 nm. Details of the construction of the applicator are given, together with measurements of the uniformity of irradiation and the light loss within the scattering medium. The device has been assessed in nine patients in a phase I trial. In two of these patients, the penetration depth of 630 nm light in brain tissue has also been measured in vivo using an optical-fibre probe, indicating that the penetration depth of 630 nm light in human brain is greater than that found in vitro.