A system of scientific classification should have a suitable basic axis of organization, standardized names, clearly specified operational criteria, and multiaxial arrangements for citing important attributes beyond those included in the basic axis. During the past century, the main nosologic system for identifying human ailments has been the International Classification of Diseases (ICD), which has a well-organized and well-accepted nomenclature, but which lacks operational criteria and an appropriate multiaxial pattern. Two new systems of classification during the past two decades are intended for other purposes and would not be satisfactory as nosologic substitutes. The Problem-Oriented Record (POR) does not have a standardized nomenclature or criteria; and the Diagnosis-Related Group (DRG) approach was organized mainly for fiscal goals. As the basic taxonomy used for classifying human ailments, the ICD needs substantial improvement to fulfill its scientific role in statistics for the occurrence and treatment of disease.