Abstract Background: To investigate whether there are some differences in Event-Related Potentials (ERP) between melancholic patients and healthy controls. To establish whether there is a relationship between abnormalities of ERP and severity of depression and psychomotor retardation. Method: Melancholic depressed patients ( N=50) and normal comparison subjects ( N=31) were assessed for latencies and interlatencies of N100, N200, N400, latency and amplitude of P300. The ERPs were studied with an ‘oddball paradigm’ in the auditory modality. Severity of depression was measured by the Hamilton Depression Rating Scale (HDRS) and psychomotor retardation with the Depressive Retardation Rating Scale (DRRS). Results: The melancholic group showed a significantly higher latency in N100 ( P<0.001), N200 ( P<0.001) and P300 ( P<0.001) and a significantly lower P300 amplitude ( P<0.001) than healthy controls. No other differences were found either in the latencies of the N400 or in their interlatencies. HDRS and DRRS do not have any significant correlations with amplitude or latency measures. Limitations: The subjects of this study are inpatients, with a severe subcategory of depression and high average age. It is difficult to generalize these findings. Conclusions: The principal finding of this study is the increase in three of the four latencies measured (N100, N200 and P300) and in the decreased P300 amplitude in melancholic patients compared to normal controls. There is no association between these abnormalities and clinical variables.