There are numerous magnetic resonance imaging (MRI) studies investigating the hippocampal volumetric differences between depressed and healthy subjects. Although there are inconsistencies among study results, a smaller hippocampal volume in depressed patients is thought to be related to the pathophysiology of the disease. Many factors appear to affect hippocampal volumes in major depressive disorder patients. Among these factors, recurrency, severity and the individual patient are the most pronounced. Patient groups with a mean age older than 40 years, or samples consisting of patients who have had severe or multiple episodes are more likely to demonstrate smaller hippocampal volumes. The possible causes of this relationship are discussed to give new perspectives to future research.