Gamma oscillations are key in coordinating brain activity and seem to be altered in schizophrenia. In previous work, we studied the spatial distribution of a noise power measure (scalp-recorded electroencephalographic activity unlocked to stimuli) and found higher magnitudes in the gamma band related to symptoms and cognition in schizophrenia. In the current study, we sought to replicate those findings and to study its specificity for schizophrenia in a completely independent sample. A principal component analysis (PCA) was used to determine the factorial structure of gamma noise power acquired with an electroencephalographic recording during an odd-ball P300 paradigm in the 250- to 550-ms window in 70 patients with schizophrenia (16 patients with first episode), 45 bipolar patients and 65 healthy controls. Clinical and cognitive correlates of the resulting factors were also assessed. Three factors arose from the PCA. The first displayed a midline-parietal distribution (roughly corresponding to the default mode network), the second was centro-temporal and the third anterior–frontal. Schizophrenia but not bipolar patients showed higher gamma noise power loadings in the first factor in comparison with controls. Scores for this factor were significantly and directly associated with positive and total symptoms in patients and inversely associated with global cognition in all participants. The results of this study replicate those of our previous publication and suggest an elevated midline-parietal gamma noise power specific to schizophrenia. The gamma noise power measure seems to be a useful tool for studying background oscillatory activity during performance of cognitive tasks.