The purpose of this study was to evaluate diagnostic MRI criteria in Wegener's granulomatosis of the nasal cavity, the paranasal sinuses and orbits. Between March 1991 and January 1996, 62 patients with biopsy-proven Wegener's granulomatosis were studied with T1- and T2-weighted spin-echo (SE) sequences. In 32 patients coronal postcontrast T1-weighted images were obtained. Mucosal thickening of the nasal cavity and paranasal sinuses was demonstrated as high-intensity lesions on T2-weighted SE sequences in 57 patients (92%). Of this group, inflammatory granulomatous tissue was found on biopsy in 30 patients (48%) in the nasal cavity and in 4 patients (6%) in the paranasal sinuses. In 23 patients (37%) biopsy revealed unspecific inflammatory changes without evidence of granulomatous tissue. In 14 patients (23%) granulomas were depicted as low-signal intensity lesions on T1- and T2-weighted SE sequences in the paranasal sinuses and orbits. In 5 patients (8%) osseous destruction was found. After gadolinium injection, 12 of 14 granulomas showed inhomogeneous signal enhancement. In two granulomas no enhancement was found. The MRI technique is helpful in the diagnosis of patients with Wegener's granulomatosis. In the initial inflammatory process of Wegener's granulomatosis, it is not possible to differentiate between mucosal inflammation and granulomatous tissue in MRI. In the later stage of granulomatous transformation, granulomas can be depicted as low-signal-intensity lesions. Therefore, Wegener's granulomatosis should be included in the differential diagnosis of patients with low-signal-intensity lesions on T1- and T2-weighted SE sequences of the nasal cavity, paranasal sinuses and orbits.