Abstract 1. 1. It is possible to diagnose cases reporting in the mixed dentition with a fair degree of accuracy regarding the possibilities of treatment with or without extraction of premolars in the permanent dentition. 2. 2. Calculations may be made from tooth and linear arch measurements to determine whether any retentive type of treatment in the mixed dentition will be successful in obtaining a slight increase in linear dimension of the permanent dentition and thus obviate reduction of tooth structure in borderline cases. 3. 3. Measurements made on mixed dentitions compared with those on permanent dentitions of the same cases years later without treatment, or after treatment and relapse, demonstrate that calculations can be made on the mixed dentition casts to practical advantage in diagnosis and treatment planning. 4. 4. Tooth measurements made on the casts of the patient reporting in the permanent dentition may be used in computing the relative discrepancy in bone and tooth structure and to assist in treatment planning by offering a working basis for determining the amount of mesial movement of the posterior anchorage units permitted in treatment procedure. 5. 5. A study of the profile and the Frankfort-mandibular angle should be made at the initial appearance of the patient, but the calculation is not affected by the findings, although they must be dealt with when the patient submits to active treatment.