The adverse human health effects associated with the most prolonged and intense exposure known to Pfiesteria piscicida Steidinger & Burkholder cultures and toxin(s) are described. In December 1993, a patient presented with acute illness to the Memory Disorders Clinic of the Bryan Alzheimer's Disease Research Center at Duke University Medical Center with significant cognitive deficits 2 weeks after ceasing occupational laboratory exposure on the recommendation of the evaluating primary care physician. The clinical and exposure histories of this patient are presented. The comprehensive neurological examination findings are reviewed, with attention to the patient's neuropsychological evaluation. Six-week follow-up data illustrate the course of symptom resolution with exposure cessation. This case is presented in an effort to contribute to the gradually accruing evidence of potential central nervous system sequelae of Pfiesteria exposure. The case is discussed in the context of additional cases evaluated at Duke University Medical Center and the complicated scientific framework in which such evaluations proceed while definitive surrogate or biological markers are awaited.