Background: Surgery is the most frequent treatment modality for basal cell carcinoma but in spite of its high cure rate, the frequency of incomplete excision varies widely (0.7-50%) among dermatologic centers. Our case series was designed to determine the frequency of incompletely excised basal cell carcinoma and the related risk factors. Methods: A total of 1424 basal cell carcinoma (1040 patients) lesions which were excised in Razi Dermatology Hospital of Tehran from 2006 to 2008 were evaluated in this case series and their findings were analyzed with SPSS software. Results: Incidence of incomplete excision was 12% and involvement of the deep margin was observed in 54% of these lesions. Factors related to incomplete excision were infiltrative, morpheic and micronodular subtypes, lesions larger than 20 mm, those repaired by skin grafts and those that received local anesthesia. There was no statistically significant difference in age, sex, site of lesions and childhood history of radiotherapy for tinea capitis. Conclusion: Recognizing the risk factors related to incomplete excision of BCCs would help us consider a wider excision margin for high risk tumors.