Presents RAND"s analysis of the benefits and costs of mandating a switch (either simultaneously or sequentially) from the International Classification of Diseases, 9th Revision, codes for diagnoses and procedures to two code sets based on the 10th revision. The author concludes that that switching to both ICD-10-CM and ICD-10-PCS is likely to generate more benefits than costs and that the switch, if it takes place, should be simultaneous.