There is increasing evidence from genetic, biochemical, pharmacological, neuroimaging and post-mortem studies that immunological dysregulation plays a crucial role in the pathogenesis of psychoses. The involvement of microglia in schizophrenia and bipolar disorder (BD) has remained controversial, however, since results from various post-mortem studies are still inconclusive. Here, we analyzed the estimated density of microglia of age-matched individuals with schizophrenia (n=17), BD (n=13), and non-psychiatric control subjects (n=17) in the anterior midcingulate cortex (aMCC), a brain area putatively involved in the pathogenesis of psychoses, using ionized calcium binding adaptor molecule 1 (Iba1)—immunohistochemistry. The microglial cells displayed a homogenously distributed Iba1—staining pattern in the aMCC with slightly varying activation states in all three groups. The estimated microglial densities did not difer signifcantly between individuals with schizophrenia, BD and control subjects. Remarkably, when both hemispheres were investigated separately within the three groups, the density was signifcantly lateralized towards the right aMCC in schizophrenia (p=0.01) and—even more evident—in BD subjects (p=0.008). This left–right lateralization was not observed in the control group (p=0.52). Of note, microglial density was signifcantly lower in BD individuals who did not commit suicide compared with BD individuals who died from suicide (p=0.002). This diference was not observed between individuals with BD who committed suicide and controls. The results, tentatively interpreted, suggest a hitherto unknown increased lateralization of microglial density to the right hemisphere in both psychiatric groups. If confrmed in independent samples, lateralization should be considered in all post-mortem studies on microglia. Density diferences between suicide and non-suicide individuals needs further elucidation.