Malignant lymphoma (lymphosarcoma) and leukemia are among the most common malignancies seen in ferrets. Diagnosis may be made by fine needle aspiration or biopsy. For a dedicated owner with a compliant patient, surgery and/or treatment with chemotherapy is an option. Remission of lymphoma is possible with treatment in ferrets, but recurrence is common. Ferrets also commonly develop insulin-producing tumors of the pancreas that lower the ferret’s blood sugar and cause weakness, weight loss, lethargy, seizures, coma, and death. Insulinoma commonly spreads from the pancreas to the liver, so surgical removal of pancreatic insulinoma nodules may not be curative. Affected ferrets respond well for months to years to medical therapy with glucose-promoting drugs (prednisone) and anti-insulin drugs (diazoxide). Drugs suppress effects of the tumor but do not eliminate it; and ferrets on medical treatment must have their medications increased over time as the tumor grows.