Progressive multifocal leukoencephalopathy (PML) is a rare demyelinating lytic brain infection caused by the John Cunningham virus (JCV). JCV manifests primarily in patients with innate immunodeficiency or taking immunomodulatory medications. In this case study, we report a PML patient with comorbid mediastinal teratoma and mild lymphopenia.A 73-year-old female presented with a 3-month history of progressive hemiplegia, hemianopsia, and cognitive impairment. She was diagnosed as PML by cerebrospinal fluid metagenomics sequencing and brain biopsy. Extensive immunological tests did not reveal an apparent immunodeficiency, but further work-up revealed that the PML was most likely the first presentation of mediastinal teratoma and the mild lymphopenia. Mirtazapine and immunoglobulin were started, the patient's condition was relatively stable and approved to be discharged from hospital. But unfortunately, she died of the lung infection 10 months after first presentation.This case confirms that mediastinal teratoma may induce the lymphopenia and trigger PML, delayed or incorrect diagnosis may worsen the course of the disease and result in poor prognosis.© 2022. The Author(s).