The effect of intercurrent brain lesions on the characteristics of chronic epilepsies has not yet been the subject of detailed investigation. We therefore retrospectively examined this issue for idiopathic, cryptogenic as well as symptomatic chronic epilepsies in patients documented in routine investigations at our outpatient seizure unit between the occurrence of epilepsy and commencement of the study. The clinical criteria for inclusion in this study were brain injury by trauma, stroke or intracerebral hemorrhage, documented by CT scan, and the occurrence of this so-called index event in patients with established chronic epilepsy. These requirements were met by 63 patients (40 male, mean age 44.8 years, SD 14.7 a; 23 female, mean age 43.1 years, SD 16.4 a). Seizure characteristics, EEG recordings as well as seizure frequencies before and after the index event were compared. Patients and controls were observed for a mean time of 22.12 years (SD 7.33 a) and 22.01 years (SD 11.31 a), respectively. Two patients presenting with generalized seizures prior to brain injury underwent a change in seizure characteristics. One control (without intercurrent brain lesion; matched for age, sex, age at onset of seizure disorder and classification of seizure disorder) primarily presenting with partial seizures evolving to generalized seizures showed changed seizure characteristics. These changes are statistically insignificant (McNemar test for the significance of changes). Seizure frequencies decreased significantly after brain injury. EEG recordings mirrored clinical findings with additional focal aspects according to brain lesion. These findings are discussed mainly with respect to the cofactorial etiology of epilepsies.(ABSTRACT TRUNCATED AT 250 WORDS)