Objective To investigate the association between the presence of bilateral synchronous rhythmic slow wave in ambulatory EEG recordings and the development of secondary epilepsy in patients with supratentorial tumor.Methods A total of 255 patients with supratentorial tumor admitted into the First Affiliated Hospital of Chongqing Medical University or the Third People′s Hospital of Chengdu from January 2008 to July 2015 were included in this study.All patients received 24 hEEG monitoring using a 16-channel ambulatory EEG system.The potential association between the presence of bilateral synchronous rhythmic slow wave in ambulatory EEG recordings and the development of epileptic seizures or the tumor location was analyzed.Results Bilateral synchronous rhythmic slow wave was detected in the ambulatory EEG recordings of 145 patients

51 of whom had a history of epileptic seizures;such a wave was not observed in the other 110 patients

26 of whom had a history of epileptic seizures.According to the results of a chi-squared test

the presence of bilateral synchronous rhythmic slow wave in ambulatory EEG recordings was significantly associated with the development of epileptic seizures（P<0.05）.In terms of the tumor location

79 patients had a tumor located in the frontal lobe

68 in the temporal lobe

42 in the parietal lobe

8 in the occipital lobe

22 in the anterior cranial fossa

6 in the middle cranial fossa（5 in the sellar region

and 1 on the sphenoid ridge）

26 had midline tumors（3 in the lateral ventricle

8 in the periventricular area

4 in the genu of the corpus callosum

and 11 in the parasagittal area）

and the other 4 had tumors in multiple locations.The detection rate of bilateral synchronous rhythmic slow wave in ambulatory EEG recordings varied significantly among patients with tumor（s）in different locations（P<0.05）.The incidence of secondary epilepsy also varied significantly among patients with tumor（s）in different locations（P<0.05）.Conclusion Bilateral synchronous rhythmic slow wave is not specific to patients with epilepsy secondary to supratentorial tumor

but patients with the wave is more susceptible to secondary epilepsy than those without such a wave.