In the past few years, non-Hodgkin's lymphomas have been paid increasing attention to because of their recently increasing frequency. We reviewed the MR images of 17 patients with histologically proved primary CNS lymphoma, all of them immunocompetent at diagnosis. We studied the site, number and shape of the lesions, the presence and grade of edema and possible periventricular spread. The exams were performed with 0.5 T and 1.5 T MR units, using SE sequences on the sagittal, axial and coronal planes, before and after Gd-DTPA administration. The most typical neuroradiologic signs which may suggest the diagnosis of CNS lymphoma are deep or periventricular lesion site, diffuse and marked contrast enhancement, poorly defined borders, moderate edema surrounding the mass and a tendency to periventricular spread. MRI demonstrated 35 lesions in 17 patients. The lymphoma was unifocal in 9 cases (53%) and 7 lesions were localized in subtentorial site. Lesion size did not exceed 2 cm in 49% of cases, ranged 2-4 cm in 40% and exceeded 4 cm in 11% of cases only. These lesions and hypo- to isointense on T1-weighted images (97%) and their signal intensity varies on T2-weighted images, with mainly iso-/hypointense patterns (79%). All lesions enhanced after Gd-DTPA administration, 74% of them markedly and 26% moderately; enhancement was mostly homogeneous (80% of cases). Perilesional edema was observed in 74% of cases. In conclusion, MRI yields some useful information for the diagnosis of primary CNS lymphoma, but the clinical and radiologic signs of this lesion may exhibit aspecific signal features, meaning that no correct diagnosis can be made even in immuno-competent patients.