Progressive multifocal leukoencephalopathy (PML) is a demyelinating disease which occurs in immunodepressed subjects and is particularly frequent in AIDS. Some authors having drawn attention to the protean aspect of the disease and claimed that AIDS may lose its basic characteristics and affect the grey matter as well as the white matter, we reviewed a series of 8 patients who had been biopsied and/or autopsied and had been examined at least once by MRI. In this series, contrary to what is regularly observed in toxoplasmic abscesses we did not find any lesion of the grey matter or any mass effect. On the other hand, we confirmed that PLM is not multifocal in all cases and that it course may be interrupted by prolonged remissions. The MRI criteria for PML therefore are reliable, provided multiple T2-weighted slices in coronal plane are performed, clearly showing the anatomy of the white fibres affected. However, it must be borne in mind that HIV-infected patients often have other associated brain pathologies, especially when the immune deficiency increases.