The Organismic Age concept has had considerable appeal to elementary teachers. Perhaps one reason for this is the fact that its proponents have put it forward in a setting of enlightened school practices. In this paper we are not concerned with these practices but with the validity of the OA concept.Long since accepted is the idea that a child's readiness for reading, arithmetic, third-grade science, and so on is a function of his mental development, total mental development. In this, the MA is the most important single component. But social development, emotional maturity, before-school experiences, language development, motor coordination, and perhaps other factors are also important. The OA is an average of age scores of mental data and age scores of anatomical and physiological data, which hereafter we shall call physical data. Commonly, OA is the average of mental age, reading age, dental age, grip age, metacarpal age, vital capacity age, height age, and weight age, although there is no hard and fast set of component measures. In a sense it combines measures of what is commonly called mental growth and measures of whatwe may call physical growth; and, because of the disproportionate number of measures of the latter kind, the average, OA, is weighted heavily in that direction. It is with the feasibility in school practice of making prediction of achievement, determining readiness, appraising achievement of pupils in terms of their anatomical and physiological development, as well as in terms of measures of mental development that we are concerned. Can we make better predictions or interpretations (which amounts to the same thing) by so doing?Obviously the first questions one would ask in this connection are: Are measures of physical growth related (1) to mental growth, and (2) to academic achievement? Moreover, if found to be related, are they related to an extent that makes any practical difference?They are, of course, related in a spurious way. All the measures that go into OA are related one to another in this way. Any growth process requires time. Childhood is the period of growth. Most of the growth functions begin in early childhood, even before birth,