Patients with exposure to Pfiesteria toxin have developed an illness, Pfiesteria-human illness syndrome, characterized by skin lesions, headache, myalgias, conjunctival irritation, bronchospasm, abdominal pain, secretory diarrhea, recent memory loss, and difficulties with number sequencing. Not all patients demonstrated all features of the syndrome. The natural history of Pfiesteria-human illness syndrome shows that most patients' symptoms improve without treatment. This article reports the improvement of symptoms that had persisted for over one month in five patients, which the author attributes to treatment with cholestyramine. These patients were self-referred to the Pocomoke River Rash and Associated Illness Center, a clinic that opened on August 6, 1997, in response to the need for a central facility for diagnosis of human illness acquired from Pfiesteria. Until the Pfiesteria toxin(s) is isolated and characterized, and laboratory diagnostic tests are available, physicians must be able to recognize Pfiesteria-human illness syndrome and intervene when symptoms, particularly memory loss and diarrhea, cause significant impairment in daily activities. There are no precedents for the treatment of Pfiesteria or any dinoflagellate toxin-related human illness reported in the literature. The successful use of cholestyramine reported here may provide a model for understanding dinoflagellate toxin physiology in the human body. This paper reports an uncontrolled observational study. When identification of the toxin is completed, a basis for properly controlled studies will be available.