Background: Observation of patients with morbilliform eruptions reveals a distinctive demarcation line on the upper arms. Objective: We studied the origin of this sharp “drug line.” Methods: We performed a literature search of all known variables. Results: The drug line represents clinical expression of the pigmentary Voigt-Futcher line. Conclusion: The drug line reveals the otherwise inapparent embryologic ventral axial line, which marks the precise border between the sensory innervation of the lateral and medial upper arm. The significance of the cutaneous sensory nerves is thus apparent. The drug line is further evidence of the segmental nature of human skin, as evidenced in evolutionary and embryologic studies.(J Am Acad Dermatol 1999;40:736-40.)