The comparative values of computerized tomography and radionuclide scan for the diagnosis of brain lesions were assessed on a series of 550 patients. A number of indications for each of these two methods have emerged from our results and from those found in the literature. The biases encountered in this type of study are discussed, and a "decision-making flow-chart" is proposed. The procedure of choice is computerized tomography in patients with suspected tumour or intracerebral haematoma, and radionuclide scan in those with suspected subdural haematoma or superficial cerebrovascular disease. The usefulness of clinical examination to determine precisely which of these two methods should be used cannot be overstressed.