: Being a subtype of primary central nervous system lymphoma (PCNSL), primary vitreoretinal lymphoma (PVRL) is a rare and fatal intraocular malignancy manifesting as blurred vision and floaters, and is usually combined with, or eventually progresses to, central nervous system lesions. The diagnosis of PVRL/PCNSL remains challenging because of the nonspecific clinical features and diagnostic dependency on biopsy.: In this paper, we present the clinical, imaging, laboratory, brain biopsy, and vitreous biopsy findings of a 56-year-old immunocompetent woman who presented with blurred vision of the left eye, but which rapidly evolved into lesions of the central nervous system. The dramatic changes on brain imaging and the undiagnostic brain and vitreous biopsy results presented great challenges for the diagnosis. PCNSL was eventually presumed according to comprehensive consideration of the disease progression pattern, the characteristic neuroimaging, and molecular hints.: PCNSL is a highly invasive tumor, and timely diagnosis is the key point in clinical practice. However, the requirement for biopsy and the existence of sentinel lesions impedes the diagnosis. Therefore, follow-up and repeated biopsy is always necessary for a definitive diagnosis. This case indicates that a complete evaluation of neuroimaging, ophthalmic testing, cytologic examination of the cerebrospinal fluid, diagnostic vitrectomy, and brain biopsy are essential for diagnosis of PCNSL. Moreover, molecular and cytokine analyses are useful adjuncts to the diagnostic cytology. Of note, the analysis of cytokine levels (IL-10/IL-6) is an important auxiliary diagnostic strategy in the diagnosis of diffuse large B-cell lymphoma.