Strategies for recognition and quantification of misclassification are presented. Misclassification occurs when a subject is placed in the wrong "exposure" or the wrong "outcome" category as a result of the failure of an index measurement to reflect exposure or disease accurately. Data from a Health Hazard Evaluation illustrate how misclassification may result in an unwarranted etiologic assumption. Data from a lung cancer screening program illustrate how misclassification may result in erroneous estimates of disease prevalence.