Changes in the human nail frequently serve as an indicator of local and systemic disease. Alterations in the morphology, structure and growth characteristics of the nail accompany chronic cigarette smoking; yellow pigmentation of the nail plate--referred to as the "nicotine sign"--is common. The clubbed yellow nail may indicate the presence of lung cancer. In contrast to the ominous nature of the clubbed yellow nail, we describe a sign that is more propitious: the sudden cessation of smoking due to an intercurrent disease, often a cerebrovascular accident (CVA), leads to the development of a distinct line of demarcation between the distal pigmented nail and the newly emerging proximal nonpigmented nail. We propose the term "harlequin nail" for this curious physical sign. By measurement of the distance between the proximal nail base fold and the line of demarcation, we can deduce the date smoking ceased (and, by inference, the approximate date of a CVA in a patient unable to volunteer this information). This sign also serves as a reminder that the "nicotine nail" remains discolored only because of dynamic restaining of the nail with tobacco by-products.