This case report describes a cachectic patient with a first hypomanic episode, hypercortisolism and a newly diagnosed large B-cell lymphoma. Hypercortisolism, causing the manic symptoms, may be explained due to low leptin levels as a result of cancer cachexia and fasting. Patients with cancer frequently have psychiatric complications, but manic symptoms are rare. To our knowledge, this is the first report of a patient with a lymphoma presenting with manic symptoms without any cerebral abnormalities.