Cerebral toxoplasmosis remains the most frequent etiology of cerebral masses in AIDS. In most cases, the disclosure of multiple enhanced masses is suggestive of diagnosis of toxoplasmosis and leads to undertake presumptive therapy. Sometimes, the pattern is less suggestive, and the possibility of primary cerebral lymphoma (PCL) is a diagnostic dilemma, because this is a short term life-threatening lesion in the absence of an undelayed fitting therapy. However, apart from the periventricular form of PCL, where directly proceeding to biopsy should be suggested, there is no distinctive pattern of PCL. The most reliable features in distinguishing between PCL and toxoplasmosis are: solitary lesion, homogeneous enhancement of a lesion whose diameter is larger than 2 cm, situation in deep periventricular white matter and limited edema and mass effect. The presence of haemorrhagic foci in the lesion, if there is no steroid therapy is rather indicative of toxoplasmosis. The lack of enhancement on enhanced CT-scan does not allow to eliminate cerebral toxoplasmosis, and indicate MR examination, or empiric antitoxoplasmic treatment. The radiologic follow-up depends on clinical response and on initial pattern. The criteria of response to antitoxoplasmic treatment are: the decrease in volume or number of lesions, the decrease of edematous reaction and mass effect (if no steroid was administered). If the lesion was initially equivocal, the lack of growth in a 8-day delay is a criterion for toxoplasmosis, just as the occurring of haemorrhage in the lesions (if there is no steroid-therapy).