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Abstract: Distinguishing hypertrophic lichen planus (HLP) and squamous cell carcinoma (SCC) can be diagnostically challenging because of overlapping clinical and histopathological features. This study characterizes histopathological features in HLP and SCC, assessing their utility in diagnosing atypical squamous proliferations. We compared 12 histopathological features of 15 HLP and 11 SCC biopsies from the lower extremities. We then reviewed 16 cases that were diagnosed as atypical squamous proliferations with differential diagnoses of HLP versus SCC. Clinical follow-up allowed for retrospective categorization of these difficult cases as HLP or SCC. HLP showed significant differences in hyperorthokeratosis (P = 0.04), wedge-shaped hypergranulosis (P = 0.0033), and irregular psoriasiform hyperplasia (P = 0.004), whereas parakeratosis (P = 0.001), solar elastosis (P = 0.001), deep extension (P = 0.02), and perforating elastic fibers (P = 0.0001) were significant for SCC. A scoring system based on these significant differences was devised to aid the classification of difficult cases. 56% of the difficult cases received an "indeterminate" score. A score favoring HLP had a sensitivity of 44% and a specificity of 71%. Although significant differences were identified between cases of definitive HLP and SCC, these histopathological features were unable to distinguish difficult cases, highlighting the need for clinicopathological correlation in patients with atypical squamous proliferations of the lower extremities. Many difficult cases had histologic features that could not be evaluated because of the superficial nature of the biopsy. Therefore, obtaining a deep wedge or punch biopsy may facilitate a diagnosis in cases with a differential diagnosis of HLP and SCC., Competing Interests: The authors declare no conflicts of interest., (Copyright © 2020 Wolters Kluwer Health, Inc. All rights reserved.)