Patients [32] with histologically proved malignant disease involving the paranasal sinuses were studied by CT [computed tomography]. The radiological features of tumor were sinus opacification, a soft-tissue mass, bone erosion and/or displacement, sclerosis and new-bone formation. Measurements of tissue densities were not helpful in distinguishing tumor from benign disease. Significantly greater tumor extent was demonstrated by CT than by conventional methods in 15 patients; the additional tumor most commonly involved the pterygoid region or orbit. The clinical importance of various directions of tumor spread is emphasized.