In Reply. —Drs Hudson, Daniels, and Horwitz suggest that the CDC surveillance definition allowed misclassification of patients with other myalgia causing illnesses (such as fibromyalgia) as having EMS, thus overestimating the magnitude of the 1989 epidemic. Misclassification is possible with any surveillance definition.1However, the criteria for EMS surveillance are quite rigorous, requiring eosinophil counts greater than 1.0×109/L in addition to incapacitating myalgia.2Although severe myalgia occurs in many conditions, an eosinophil count greater than 1.0×109/L is unusual. We believe the strict surveillance definition minimized overreporting, and has probably led to substantial underreporting of EMS. The logistics of public health surveillance typically do not allow extensive medical record review by public health officials.1However, we agree that more specific, analytic investigations of tryptophan use and EMS should include careful evaluation for preexisting conditions involving myalgia and eosinophilia. In the 1990 investigation in South