PURPOSE: The differential diagnosis between brain abscesses and necrotic tumors such as glioblastomas and metastases is often difficult by conventional MRI. The goal of our study was to evaluate the ability of the diffusion-weighted (DW) MRI to differentiate these lesions from each other. MATERIALS AND METHODS: MRI was performed in 19 patients (four abscesses, seven glioblastomas, two anaplastic astrocytomas, six metastases). In addition to standard MR sequences, trace DW imaging and apparent diffusion coefficient (ADC) maps were performed. ADC values were calculated for all lesions. For statistical analysis student t-test was used. A p value less than 0.05 indicated a statistically significant difference. RESULTS: The central portion of all six metastases and eight of nine high-grade gliomas showed unrestricted diffusion, whereas all four abscesses showed restricted diffusion (low ADC values) in their cavity. However, restricted diffusion also was found in one glioblastoma. The ADC values of abscesses (0.69-/+0.05 x103 mm2/s) were significantly lower than of 15 malignant necrotic tumors (2.39-/+0.63 x 103 mm2/s). The difference between abscess and necrotic tumors was statistically significant (p<0.001). CONCLUSION: DW MRI can be used to identify a brain abscess and can help to differentiate it from a cystic brain tumor. However, restricted water diffusion is not specific and pathognomonic in the differential diagnosis between abscesses and necrotic tumors.