Abstract Background : The carbon dioxide laser is an effective modality for the treatment of actinic cheilitis, but the number of passes required is unknown. After each pass different visual tissue qualities are observed. Objective : Our purpose was to identify and characterize histologically the tissue zones seen after laser impact and thereby to determine the optimal depth of destruction of diseased tissue. Methods : Twenty-three biopsy specimens from 14 patients were obtained from zones of different tissue qualities after one and three passes with the laser. Specimens were histologically examined for the presence or absence of diseased epithelium and degree of thermal necrosis. Results : Complete destruction of the epithelial layer was observed in all specimens irrespective of the number of laser passes. The amount of dermal homogenization increased with multiple passes. Conclusion : Treatment to the first or second surgical zone is effective for actinic cheilitis.