Two patients with squamous cell carcinoma of the head and neck are reported in whom the syndrome of inappropriate antidiuretic hormone (SIADH) secretion occurred transiently during the rapid cytolytic phase of tumor destruction after chemotherapy with cis-platinum diamminedichloride and bleomycin. Immunoperoxidase staining for ADH of the original biopsy specimens from both patients was negative. Possible mechanisms for and the implications of the production of SIADH in this setting are discussed.