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BACKGROUND Shave biopsy may not be able to accurately distinguish squamous cell carcinoma in situ (SCCIS) from invasive squamous cell carcinoma (SCC). Information on the incidence of biopsy-proven SCCIS upstaged to SCC after a more complete histologic examination is limited. OBJECTIVE To determine the incidence and clinical risk factors associated with upstaging the biopsy diagnosis of SCCIS into invasive SCC based on findings during Mohs micrographic surgery (MMS). METHODS All MMS cases of SCCIS performed between March 2007 and February 2012 were identified, MMS operative notes were examined, and invasive dermal components were confirmed by the MMS slide review. Upstaged SCCIS was defined as biopsy-diagnosed SCCIS subsequently found to be an invasive SCC during MMS. RESULTS From 566 cases with the preoperative diagnosis of SCCIS, 92 (16.3%) cases were SCCIS upstaged to SCC. Location of ears, nose, lips, and eyelids, preoperative diameter >10 mm, and biopsy report mentioning a transected base were significant predictors of upstaged SCCIS. CONCLUSION Considering the possibility that over 16% of SCCIS may be truly invasive SCC, biopsy-proven SCCIS should be treated adequately with margin-assessed treatment modalities such as surgical excision or Mohs surgery when indicated.