A study was conducted to determine the causative lesion of Marek's disease virus-induced transient paralysis. Brain histologic lesions were quantitated and compared in genetically resistant and susceptible inbred chickens. Comparisons were made 11 days post inoculation (PI) of virus when clinical signs were evident in susceptible birds and on day 17 PI when these signs had remitted. Clinically-affected birds from the susceptible line on day 11 PI had consistent evidence of neuropil vacuolation compatible with brain edema; two of these birds also had vasculitis. These lesions were absent in both clinically-recovered birds from this line evaluated on day 17 PI and genetically resistant birds studied on day 11 PI. Our findings suggest that the causative lesion of transient paralysis is vasogenic brain edema.