The number of hand-wrist centers ossified has been counted in 1,447 radiographs of Guatemalan boys and in 2,130 radiographs of United States boys. It has been demonstrated that, between birth and 7 years, this simple procedure can provide considerable information about the actual centers ossified. Consequently, it can be used to estimate the stage in the sequence of onset of ossification attained by a particular boy. It has been shown that, during the age range considered, this method compares favorably with those of Greulich and Pyle, and Tanner et al. in regard to the information provided. It also compares favorably in rcgattl to the time required for an assessment and the difficulties of standardization. There is no reason why the method would not be equally suitable for girls. It must be stressed that, in hoys, its usefulness is restricted to the age range from birth to about 7 years