A study of febrile seizures in 110 patients was made with the following results: (a) incidence approximately 100% greater in boys and in whites over Negroes and Spanish-Americans; (b) most common onset between 6 months and 3 years of age; (c) cause associated with acute tonsillitis, pharyngitis or otitis media; (d) clonic type of seizure pattern in 84% of 96 patients; (e) recurrent attacks of disorders in 35% of patients; (f) history of possible brain injury in 58%; (g) 20% with a history of allergy; (h) family history of febrile seizures in 30%; (i) neurological abnormalities in 20%; and (j) seizure discharges were noted in 24% of 76 patients. Etiology, mechanism and threshold are related to body temperature, age, sex, infection, allergy, water and electrolyte balance, heredity and acquired brain injury.