Background: Alterations of mental status are characteristic of psychiatric disorders but may also result from a multitude of organic causes. Generally, physical examination and blood analysis are a part of basic psychiatric differential diagnostics, whereas more sophisticated procedures (for example, brain imaging) are applied only in cases with pathologic diagnostic findings. Our report challenges this approach by describing a case of glioblastoma multiforme presenting as postpartum depression without abnormalities in basic differential diagnostics. Case presentation: A 28-year-old white woman who had been in outpatient treatment for postpartum depression was taken to the psychiatric emergency room. The psychopathological assessment, however, showed mild disorientation and severe deficits of long-term memory. Moreover, she complained of stabbing, bilateral headaches, but results of her physical examination and blood analysis were unremarkable. Magnetic resonance imaging of the brain was performed, which showed a contrast-enhanced mass lesion in the left frontal lobe. The patient underwent urgent tumor resection, and histologic results revealed an IDH-mutant glioblastoma multiforme. The patient was discharged with a substantially improved psychopathology and without neurological deficits. Conclusions: This report adds to the evidence that postpartum depression may have organic causes in some cases, a fact that needs to be considered in the clinical setting. Atypical neurocognitive findings in a psychiatric interview may alone justify brain imaging, despite normal physical examination and blood analysis results.