A patient with a vipoma of the pancreas and persistently elevated blood levels of vasoactive intestinal polypeptide (VIP) had watery diarrhea, hypokalemia, and achlorhydria (WDHA syndrome). In the untreated state, the diarrhea was never profuse. Fecal volumes ranged from 0.16 to 1.24 L/day. Attempts to correct the dehydration by fluid and electrolyte loading resulted in a massive increase in fecal water and electrolyte loss. Prednisone cured the diarrhea and was associated with a decrease in plasma VIP levels. The patient had a marked circulatory disturbance with systemic arterial hypotension and cutaneous vasodilation that caused a subnormal body temperature. Removal of the tumor led to a dramatic change in the patient's circulation. Generalized vasodilation with systemic venous and arterial hypotension gave away to vasoconstriction with severe venous and arterial hypertension. Central venous pressure rose from -4.4 to +4.0 cm H2O and arterial pressure rose from 80/55 to 195/110 mm Hg. These changes might explain the unexpected and sometimes fatal heart failure that has complicated the removal of these tumors from some patients.