Introduction:Mohs micrographic surgery technique allows for complete margin analysis of skin tumors, which explains its lower recurrence rates over conventional surgery. Although it is known that routine processing of excision specimens represents less than 0.5% of the margins, a direct comparison with micrographic technique has not been performed so far.Objective:To compare the margins of nonmelanoma skin cancers excised conventionally, processed with serial transverse cross-sectioning ("bread-loafing"), and had negative margin readings, against the margins obtained through micrographic technique from the same tumors.Materials and Methods:Retrospective, descriptive, historical cohort study. Inclusion criteria as follows: Patients who underwent conventional excision for nonmelanoma skin cancers between 2010 and 2013 in our dermatology department and had negative margin readings. Samples were dewaxed and processed with the 3-dimensional Mohs micrographic technique.Results:One hundred one basal cell carcinomas and 26 squamous cell carcinomas were analyzed. Thirteen positive fragments were obtained, which corresponded to 11 tumors and patients; therefore, 8.7% of patients were given a false negative result in their original study. Lateral margins were more commonly affected (81.8%). There was no clear association between false negative results and histopathologic type or subtype.Conclusions:Our study is the first of its kind in terms of methodology. The "bread-loafing" technique can incorrectly report the state of the margins of surgical pieces excised by conventional surgery. Mohs' micrographic technique is superior when it comes to evaluating margins and should be regarded as the gold standard.