Fractal dimension of the electroencephalographic (EEG) signal has been argued to reflect the complexity of the underlying brain processes. To this date, conventional studies of EEG in mood disorders have not been able to distinguish between patients and normal individuals. Here we show that, compared to normal subjects, EEG fractal dimension is significantly augmented in the manic episode of bipolar mood disorder type I (BMD I) with an incremental trend towards the right hemisphere. In addition, EEG power spectrum analysis showed that in the delta bandwidth, signal amplitude increases significantly from left to right hemisphere in patients. It is concluded that during the manic episode of BMD I, asynchronous neuronal activity increases in multiple cell assemblies predominantly located in the right hemisphere. As a psychopathological hypothesis, this conclusion encompasses clinical features of the manic episode, studies of post-traumatic mania and the relevant neuroimaging findings. Implications of our findings in understanding the neurophysiological basis of the converging trends in the pharmacology of epilepsy and mood disorders are also discussed.