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Introduction: Lichen planus is a chronic autoimmune inflammatory disorder. At present, there is a lack of any specific scoring system to judge the severity of cutaneous lichen planus. Hence, a study was undertaken to establish and validate a system to define the severity of cutaneous lichen planus, i.e. Lichen Planus Severity Index., Materials and Methods: Setting: Skin outpatient department, Krishna Institute of Medical Sciences, Karad., Model: The formulation model was Psoriasis Area Severity Index (PASI) and the validation model was Onychomycosis Severity Index (OSI)., Participants: The consensus group included two dermatologists and two dermatology residents with special interest in lichen planus and a statistician. Results of the consensus group were compared with a preliminary reproducibility group of two dermatologists and four dermatology residents. Later, reliability assessment was carried out by two groups: 1. Twenty-one dermatologists scored 20 photographs of four patients of lichen planus after being trained to use Lichen Planus Severity Index. 2. Six doctors (three experts and three randomly selected physicians) evaluated ten real-world patients of lichen planus in skin outpatient department. The physicians were blind to the scores assigned by experts., Steps to Calculate Score: There are five morphological types of lesions seen in lichen planus, namely, erythematous papule, violaceous papule, violaceous plaque, hyperpigmented hypertrophic papule and plaque and postinflammatory hyperpigmentation. Total involved body surface area is determined and a body surface area factor is assigned. Area involvement factor for each of these morphological lesions is calculated and multiplied with the respective multiplication factor. Sum of all the products gives the lesion severity score. Product of lesion severity score with the body surface area factor gives the final Lichen Planus Severity Score., Results: There was no significant difference between the scores of consensus group and preliminary reproducibility group. Both assessment groups showed high reliability. (Group 1: Cronbach alpha = 0.92, ICC = 0.85; Group 2: Cronbach's alpha = 0.99, ICC = 0.92). The correlation between Lichen Planus Severity Index and the standard Physician Global Assessment score was found to be positive (correlation coefficient = 0.73)., Limitations: : The system is tedious and requires a steep learning curve. Possible uses of Lichen Planus Severity Index are yet to be explored and validated., Conclusion: Lichen Planus Severity Index is a new reproducible tool to grade the severity of lichen planus., Competing Interests: None