Viral Meningitis: This condition is characterized by an elevated white blood cell (WBC) count in the cerebrospinal fluid (CSF) with a predominance of lymphocytes. The presence of 90% lymphocytes, some of which are reactive or transformed, supports this diagnosis, as viral infections often induce a lymphocytic response.

Tuberculous Meningitis: Although less common than viral meningitis, tuberculous meningitis can also present with lymphocytic pleocytosis in the CSF. The diagnosis might be considered, especially in individuals with risk factors for tuberculosis or in endemic areas.
Lymphomatous Meningitis: This condition involves the spread of lymphoma to the meninges and can cause an elevated lymphocyte count in the CSF. The presence of transformed lymphocytes could suggest this diagnosis, particularly in patients with a known history of lymphoma.
Chronic Meningitis: Chronic meningitis, which can be caused by various pathogens including fungi, mycobacteria, and certain bacteria, may also present with lymphocytic pleocytosis. The chronic nature of the infection can lead to a prolonged elevation of lymphocytes in the CSF.

Subarachnoid Hemorrhage: Although the CSF profile does not directly suggest subarachnoid hemorrhage, this condition can sometimes present with an elevated WBC count due to the body's response to blood in the subarachnoid space. Missing this diagnosis could be catastrophic due to the need for urgent intervention.
Bacterial Meningitis (Partially Treated): Bacterial meningitis typically presents with a predominance of neutrophils in the CSF. However, in cases where the patient has been partially treated with antibiotics, the CSF profile may show a shift towards lymphocytosis. This diagnosis is critical to consider due to the high morbidity and mortality associated with bacterial meningitis.
Central Nervous System (CNS) Sarcoidosis: CNS sarcoidosis can cause lymphocytic meningitis, and while it is less common, it is a diagnosis that should not be missed due to its potential for significant morbidity if left untreated.

Leptomeningeal Carcinomatosis: This condition involves the spread of cancer to the meninges and can cause an elevated lymphocyte count in the CSF, although it more commonly presents with malignant cells in the CSF.
CNS Lymphoma (Primary): Primary CNS lymphoma is a rare form of lymphoma that arises within the central nervous system and can cause lymphocytic pleocytosis in the CSF.
Neurosyphilis: In its various forms, neurosyphilis can cause an elevation of lymphocytes in the CSF, although it is less common in the modern era due to the availability of effective treatments for syphilis.