Patients with AIDS are prone to developing infections with opportunistic pathogens. Recently, a new mycobacterium, Mycobacterium genavense, has been found to cause infection in patients with AIDS. Previously published reports indicate that patients who are infected with this organism present with the same clinical features as do patients with disseminated infection due to organisms of the Mycobacterium avium complex. We describe an unusual case of a patient with AIDS who presented with grand mal seizures and a mass lesion in his brain, which was found to be caused by infection with M. genavense. No evidence of disseminated infection could be found in this patient. We discuss the microbiology of this organism and review the literature on M. genavense infections. Clinicians should be aware of this organism so that efforts at culture and identification will be made.