Malignant gliomas accumulate fluorescing protoporphyrin IX intracellularly after exposure to 5-aminolevulinic acid, a metabolic precursor of haem. This phenomenon has been exploited for intraoperative identification of residual tumour to enable greater completeness of tumour removal. The present report describes the necessary modifications to the operating microscope to enable microsurgical, fluorescence-guided tumour removal. The system consists of a xenon light source coupled to the microscope, which can be switched from normal white light to violet-blue excitation light (375-440 nm). A longpass filter is introduced into the observer light path to enable observation of tumour fluorescence. Transmission characteristics of excitation and observation filters are chosen to transmit part of the remitted excitation light. Thereby the observer retains an impression of tissue detail, next to tumour porphyrin fluorescence. An integrating three chip CCD camera optimized for red light detection enables documentation of fluorescence findings. The present modifications allow uncomplicated and rapid recognition of red tumour fluorescence and its borders to normal tissue, without interrupting the course of the operation. Tissue detail is great enough to enable tumour resection under violet-blue excitation light during parts of the operation. The system appears to constitute a useful tool for optimizing removal of malignant gliomas on a routine basis.