A revolutionary change in the reporting of biologic measurements has gradually been evolving. We asked Edward Powsner, MD, to provide our readers with a commentary on these changes. All of the American Medical Association journals will be using the following schedule in the transition from current units to the SI: Beginning in July 1986 we will report the old units with the SI units in parentheses; in July 1987 we will report the SI units first with the old units in parentheses; and from July 1988 onward we will report only SI units.—Ed.