When is lichen planus lichen planus.

T ERMATOSES of diverse etiology have been -I-~ reported to give rise to skin and mucous membrane lesions whose morphology, symptomatology, course, and microscopic picture cannot be distinguished from lichen planus. Drugs,1-6 vaccines,7 contact with chemicals,8-'3 and exposure to ultraviolet light and Grenz rays14 are prominent among the causative agents which have been observed to produce lichenoid or lichen planus-like eruptions. The cause of lichen planus is not known, but the muco-cutaneous lesions produced by these agents can simulate it in minute detail. Could lichen planus then represent a symptom complex or disease which can result from diverse causes in susceptible individuals? It is the purpose of this paper to discuss this possibility.