We report two patients who presented with altered mental status and involuntary movement under administration of cefepime (CFPM). Both cases had impaired renal function. Electroencephalogram (EEG) revealed generalized periodic discharges (GPD) or generalized rhythmic delta activity (GRDA): 3Hz in the case 1 and 1 to 2Hz in the case 2. In spite of the severe abnormality in the EEG, both patients could respond to simple commands, which suggested possible dissociation between clinical findings and EEG. The patients recovered several days after discontinuation of CFPM, and had no clear residuals. We sometimes have difficulty in determining the cause of non-specific altered mental status; drug-induced, toxic, or metabolic encephalopathies are possible differential diagnoses. Arguably, the EEG findings shown here can be a clue to correct diagnosis of CFPM induced encephalopathy.