We report a prospective clinical and electroencephalographic study of 19 patients with juvenile myoclonic epilepsy and absence seizures. Absences began 1 to 9 (4.5 +/- 2.5) years before myoclonic jerks and generalized tonic-clonic seizures. Clinical manifestations during the absence ictus showed great variation, ranging from subtle or no overt features to severe impairment of consciousness, and severity was age related. Simple and complex absence seizures can occur in the same patient. The electroencephalographic features were distinct, with many interictal discharges, fragmentation of the paroxysms, and frequent polyspikes of varying numbers and amplitude for each spike-slow wave component. The combined clinical-electroencephalographic manifestations were characteristic and allow differentiation of absences in juvenile myoclonic epilepsy from typical absence seizures in other epileptic syndromes.