Leukoderma is a generic term for any pigmentary dilution, be it congenital or acquired, circumscribed or generalized, devoid of or partially lacking in pig- mentation. In the approach to the diagnosis of leukoderma, we have generally first considered the age of onset, whether leukoderma was congenital or ac- quired, the extent and pattern of involvement, and the degree of pigmentary dilution. The organization of this monograph reflects this approach. For ex- ample, we have separated the section devoted to various disease entities into diffuse and circumscribed leukoderma and the latter into various etiologies such as genetic, metabolic, infectious, and endocrinologic. One of several justifications for this monograph is to present an approach to the diagnosis of leukoderma, as detailed in Part II. In formulating a guide for the physician, we have found some limitations to our previous approach; we therefore offer the following new classification based upon a clini- cal-pathologic correlation. This could provide the means to describe both the clinical and pathologic findings in one term.