Objective: To update some of the clinical features of St Louis encephalitis (SLE), a common arboviral infection that occurs in epidemic patterns in the southcentral and midwestern United States. Methods: Eleven patients with SLE from a 1995 epidemic in Dallas, Tex, were studied clinically, radiologically, neurophysiologically, and neuropathologically (in 1 case). Results: The electroencephalograms and magnetic resonance imaging (MRI) scans of our patients revealed features that have received little attention in previous studies. Of the 9 patients who were examined with electroencephalography, all 9 had seizures or other abnormalities, and 1 had nonconvulsive status epilepticus. Two of 6 patients who had MRIs showed substantia nigra edema. Finally, 2 (18%) of our patients had coinfection with the human immunodeficiency virus. Conclusions: The MRI findings of substantia nigra edema in patients with SLE have not been previously reported. Nonconvulsive status epilepticus can occur in patients with SLE and should be considered in patients with prolonged encephalopathy. Finally, human immunodeficiency virus coinfection may be a risk factor for symptomatic SLE infection.