Acute lymphoblastic leukemia and other aggressive lymphoid malignancies like Burkitt leukemia/lymphoma have high incidence of central nervous system (CNS) involvement. Various solid tumors, most notably breast cancer, can also metastasize into the CNS as a late stage complication causing devastating effects. Intrathecal (IT) chemotherapy consisting of methotrexate, cytarabine, or the two in combination is frequently used for the prophylaxis and treatment of CNS metastasis. Because of the high toxicity of these chemotherapeutic agents, however, their side effect profiles are potentially catastrophic. The incidence of neurotoxicity secondary to IT chemotherapy is well defined in the pediatric literature but is poorly reported in adults. Here, we investigated the incidence of neurologic and nonneurologic side effects secondary to IT chemotherapy in 109 consecutive adult patients over a two-year time period at hospitals associated with our institution. Of 355 IT chemotherapy treatments received by these patients, 11 (3.10%) resulted in paresthesias or paralysis, which we defined as significant neurologic events in our analysis. We also examined minor events that arose after IT chemotherapy, including back pain, headache, fever, vomiting, and asthenia. At least one of these occurred after 30.70% of IT chemotherapy doses. Clinicians involved in the care of patients receiving IT chemotherapy should be aware of these findings and consider treatment options lower rate of neurotoxicity such as high-dose systemic methotrexate.