Case reports and animal models suggest that chemotherapy, corticosteroids and radiotherapy (RT) may increase the risk of herpes simplex encephalitis (HSE). We retrospectively examined cases of HSE at an academic hospital devoted to cancer care. Patients were identified by positive herpes simplex virus (HSV) polymerase chain reaction (PCR) in cerebrospinal fluid (CSF) or by brain pathology. There were seven patients with HSE over a 12 year period, four of whom had received cranial RT. During this time, a total of 997 patients were treated with cranial RT, suggesting a greater incidence than the expected risk of two to four cases per million people per year in the general population. Five patients had recently received chemotherapy and three were on dexamethasone. MRI findings were typical; four patients had bilateral anterior temporal lesions and three had unilateral-temporal lesions. Four patients had a normal CSF white blood cell count, three of whom had prior RT and dexamethasone. Four patients were positive for HSV-1, and two for HSV-2. One patient had a negative CSF PCR for HSV, but autopsy confirmed active HSE. Though still rare, the risk of HSE may be increased in patients with cancer, especially in those receiving cranial RT. MRI findings were typical, but CSF white blood cell count was normal in four patients and one had negative CSF testing, suggesting that CSF results may be misleading in this population.