This study tested the hypothesis that the duration of fever prior to the onset of a simple febrile seizure may be an important clinical variable with respect to patient outcome. The duration of fever prior to seizure according to patient history was defined as either long (greater than or equal to 24 hours) or short (less than 24 hours). We hypothesized that simple febrile seizures which occur with a history of a fever of long duration (LDF) are more likely to be associated with a significant illness at presentation or a subsequent neurologically abnormal course than are simple febrile seizures which occur with a history of a fever or short duration (SDF). Of 100 cases which met study criteria for simple febrile seizures, nine had a LDF and 91 had a SDF prior to the development of a seizure. No statistical differences in age, sex, maximum fever recorded in the emergency department, duration of seizure, WBC, or electrolytes were found between patients with SDF and LDF (P less than 0.01). Of the nine patients with a LDF, all had either a significant illness at the time of initial visit or a subsequent neurologically abnormal course. Of the 91 patients with a SDF, 88 had a good outcome, while two had a significant illness at the time of visit, and one had a subsequent neurologically abnormal course. These results suggest that children with a history of LDF prior to the occurrence of a simple febrile seizure are more likely to have a serious illness at presentation or a subsequent neurologically abnormal course than are children with seizures which occur with a history of SDF.