Objective: To determine changes in clinical manifestations and cerebral blood flow (CBF) before and after administration of ECT to patients with catatonia due to schizophrenia or mood disorders. Methods: A sample of nine patients who met DSM-IV criteria for catatonia was studied. Patients received between 5 and 15 sessions (thrice per week) of ECT. Severity of catatonia was measured with the Modified Rogers Scale (MRS). Changes in CBF were measured with a brain single positron emission computer tomography (SPECT) that was performed 1 week before the first ECT and 1 week after the last ECT. Results: Catatonia was due to schizophrenia in five patients and mood disorders in four patients. There were no significant clinical and brain SPECT differences between these two groups before treatment. Pre- vs. post-ECT comparisons showed significant reduction of catatonic symptoms in both groups. However, patients with mood disorders needed less ECT sessions and showed greater clinical improvement. Brain SPECT showed significant increase in CBF in parietal, temporal, and occipital regions in patients with mood disorder and no significant changes in patients with schizophrenia. Conclusions: These results support the efficacy of ECT for treatment of catatonic patients, especially secondary to mood disorders, which seem to be correlated with improvements in CBF.