Giardia lamblia infestation reveals underlying Whipple's disease in a patient with longstanding constipation. Whipple's disease is an uncommon disorder, generally associated with gastrointestinal symptoms; of these, diarrhea is a common feature. We report a case of Whipple's disease associated with chronic constipation which was not diagnosed until after Giardia lamblia infestation had caused diarrhea. To the best of our knowledge this association has not previously been reported. The clinical, laboratory, endoscopic, and manometric aspects are described and discussed. 