This study evaluated the usefulness of diffusion-weighted (DW) magnetic resonance imaging (MRI) and ADC maps in the differential diagnosis of brain abscesses from cystic or necrotic neoplasms. MR images of 49 patients with 54 lesions were examined retrospectively. All patients underwent conventional MRI and DWI, and ADC values were calculated by placing ROIs of 30 mm(2) manually over the cystic part of the lesions. On DWI, all cystic portions of abscesses were hyperintense. Mean ADC values were 0.48×10 mm(2)/s (range 0.41-0.54×10 mm/s) for pyogenic abscesses, 0.73×10 mm(2)/s (range 0.65-0.91×10 mm/s) for mycotic abscesses and 0.6 mm(2)/s for Nocardia abscess. Cystic areas appeared hypointense on DWI in 33/44 tumours (mean value ADC 1.96 mm(2)/s). Eleven tumours (11/44) appeared hyperintense on DWI: eight metastases from lung cancer (mean ADC value 0.86 mm(2)/s, range 0.75-1.2 mm(2)/s), two GBMs (mean 0.7 mm(2)/s, range 0.67-0.76 mm(2)/s) and one anaplastic astrocytoma (ADC value 1.24 mm(2)/s). ADC values may help in differentiating pyogenic abscess from brain tumors or metastatic lesions.