Background: Hyponatremia is a known complication of cytotoxic treatment. We observed this side effect in a patient treated with bortezomib. This paper gives an overview of the literature on antineoplastic agents that have been associated with hyponatremia. Case Report: A 77-year-old female patient with mantle cell lymphoma was admitted with rapidly progressive ataxia, slurred speech, and confusion. 43 days earlier, a second-line treatment with the proteasome inhibitor bortezomib had been started. Neurological examination revealed no focal deficits. Laboratory evaluation showed a combined electrolyte disorder with severe hyponatremia (sodium 112 mmol/l). Results: A syndrome of inappropriate secretion of antidiuretic hormone (SIADH) was diagnosed, and bortezomib was identified as its cause. The drug was consecutively stopped. CT scan showed a complete remission (CR). Since, the patient has remained in a CR without further tumor-specific treatment. Conclusion: Hyponatremia may be a side effect of treatment with bortezomib and a number of other antineoplastic agents. Because of limited data available, accurate incidences of this complication are not known.