Undifferentiated carcinoma of unknown primary site complicated by lactic acidosis has not been documented. We describe a young female with undifferentiated carcinoma of unknown primary site manifested by widespread lymph node and hepatic infiltration, hyperuricemia and very high levels of lactate dehydrogenase. She developed lactic acidosis suddenly after an episode of bleeding following nasal biopsy. The bleeding episode is likely to have caused subclinical hepatic hypoperfusion and hypoxemia, thereby aggravating lactate overproduction by tumor cells and clearance impairment due to diffuse hepatic infiltration to result in rapidly fatal acidosis before cytotoxic agents could be instituted. Although uncommon, when a critical event occurs in aggressive malignancies with massive hepatic involvement, the clinician should be alert for the development of lactic acidosis because the life-threatening metabolic complication is best avoided by prompt and effective cytoreduction therapy.