In this report, we describe a 50-year-old woman with a short bowel who had recurrent episodes of weakness, ataxia, slurred speech, confusion, and nausea. D-Lactic acidosis was diagnosed on the basis of a D-lactate level of 8.2 mmol/L (normal, 0 to 0.25) obtained during an episode of confusion. D-Lactic acidosis is a potentially fatal clinical condition seen in patients with a short small intestine and an intact colon. Excessive production of D-lactate by abnormal bowel flora overwhelms normal metabolism of D-lactate and leads to an accumulation of this enantiomer in the blood. This disorder provides insight into the role of intestinal flora in human metabolism and demonstrates the manner in which altered intestinal flora can produce disease in humans. Increased awareness of D-lactic acidosis is necessary for prompt and appropriate treatment. The pathophysiology and treatment of D-lactic acidosis are reviewed.