Subacute sclerosing panencephalitis (SSPE) is mainly thought of as a disorder of childhood and adolescence and may not be readily recognized when presenting later in life. Prior reports have suggested that adult-onset SSPE may have atypical features. We have added two cases to the existing literature on adult-onset SSPE, compared them with a more classic juvenile presentation, and extensively reviewed those reports that were published after the etiological link with the measles virus had been established. Adult-onset SSPE patients present at a mean age of 25.4 years (range 20-35 years). They have a higher proportion of either negative history of measles exposure or undocumented history by the reporting authors. Those with available history of measles exposure tend to have it either earlier (younger than 3 years old) or later (after 9 years) than the usual childhood measles infection. Where the primary infection is known, unusually long measles-to-SSPE intervals have been documented, ranging from 14 to 22 years. None of the cases followed measles vaccination. Visual symptomatology was very frequent, with 8 of the 13 cases reviewed having a purely ophthalmological presentation; only 2 patients presented with behavioral changes. Although the course of the disease was progressive and fatal in the majority, there appeared to be a higher rate of spontaneous remission as compared with childhood-onset SSPE. Myoclonus, spastic hemiparesis, bradykinesia, and rigidity were the predominant motor manifestations. Neuropathology revealed cortical and subcortical gray matter involvement preferentially of the occipital lobes, thalamus, and putamen. The importance of recognizing the spectrum of potential presentations of SSPE and providing an early diagnosis will increase as more effective treatments become available.