INTRODUCTION We established a series of exercises that evaluated surgeons' marking of excision margins, and we sought to identify factors influencing such marking. METHODS Twenty-four participants were asked to draw preset margins (3, 4, 5, 8 or 10 mm) on a series of life-size images representing noncosmetically and cosmetically sensitive facial sites, and also to draw circles of set diameters (3, 5 and 8 mm) on white paper. Margins were measured with vernier callipers calibrated to 0.05 mm. RESULTS In the small margin (3 mm) and noncosmetically sensitive exercises, the mean margins drawn were greater than required. When a 10 mm margin was required in cosmetically sensitive areas and nonsensitive areas, the margin was consistently underestimated in the former group by all participants (p < 0.05). CONCLUSION Surgeons marking facial lesions for excision should use a measurement of scale, in order to eliminate the inherent tendency to underestimate the margin required for large excisions and for cosmetically sensitive areas.