“Mr. C” is an 80-year-old man who presented with new-onset generalized seizure disorder followed by gradual onset of parkinsonism. Brain MRI was notable for age-consistent parenchymal atrophy and extensive deep white-matter chronic small-vessel ischemic changes. EEG showed epileptiform activity in the left temporal lobe. A paraneoplastic panel noted positive neuronal voltage-gated potassium-channel antibody 1.15 nmol/liter, consistent with limbic encephalitis. Intravenous immunoglobulin therapy (IVIG) led to mild cognitive improvement. However, he soon developed paranoid delusions, labile mood, crying spells, and physical aggressiveness toward his wife, roughly grabbing her arm or scratching her. Plasmapheresis treatment led to mild improvement in attention and memory. However, he experienced recurrence of labile mood, crying, paranoid delusions, agitation, and a new symptom of increased libido. He was more sexually demanding of his wife and even went to the neighbors to complain about his wife's lack of interest in sex. He received a single treatment with rituximab, followed later that evening by increasingly expansive mood, decreased need for sleep, increased sexual desire, and increased aggression with his wife, accusing her of having a sexual relationship with their female neighbor. He was admitted to a subacute dementia unit and started treatment with Depakote, titrated to 500 mg at bedtime, with a trough level of 37. He had no further agitation or hypersexuality; he continued Depakote treatment, and his mood returned to normal over a few days. Mr. C had a typical case of limbic encephalitis with new-onset seizures and memory and behavioral changes without striking neurologic findings.1 Limbic encephalitis is typically paraneoplastic, but there are non-paraneoplastic forms, including voltage-gated potassium-channel antibody-associated encephalopathy, which is typically responsive to immunotherapy,2 although this case was not. Rituximab is a monoclonal antibody that binds to CD20 on B-cells, causing cell death in CD20+ B-cells and down-regulating the B-cell receptor, eliminating B-cells and thus allowing for new B-cell production. Rituximab has been successfully used in B-cell lymphoma and other autoimmune diseases, such as systemic lupus erythematosus,3 as well as paraneoplastic neurologic syndromes.4 We report a brief manic episode in the hours after rituximab infusion, a reaction that has not been previously reported. Autoantibody levels have been used to monitor outcome with IVIG in other autoimmune diseases, and might have been useful in monitoring response to therapy in this case. There are no reports of rituximab directly causing delirium5 or mood disorders, although mental-status changes have been associated with rituximab's induction of opportunistic infections such as progressive multifocal leukoencephalopathy. In short, we present a remarkable case of a brief manic episode after rituximab treatment in a patient with non-paraneoplastic limbic encephalitis. The clinician should be aware of the possibility of such a reaction, and this finding adds to our knowledge of psychiatric complications of limbic encephalitis.