SummaryBackground: This study aimed to define the anatomy of the eyelash follicles in order to improve methods of selective lash ablation.Materials and Methods: Dimensions of eyelash follicles were determined from histological specimens of the upper and lower lids of 10 patients. For one of these patients, the eyelash follicles were categorized as quiescent or actively growing, according to standard hair-cycle morphology.Results: Follicle dimensions were (mean ± SD): upper lid follicle depth, 1.8 ± 0.3 mm; bulb width, 188 ± 44μ shaft width, 205 ± 28 μm; lower lid follicle depth, 0.9 ± 0.2 mm; bulb width 132 ± 19 μm; and shaft width, 158 ± 26 μm. The upper lid comprised 41% active follicles compared to 15% in the lower lid.Conclusions: There are significant differences in the anatomy of the follicles between the two lids. For an electrolysis needle to completely contact 95% of all follicles, it must be inserted 2.4 mm into the upper lid and 1.4 mm into the lower lid (mean depth +2 SD). Argon laser ablation requires a beam width of <200 and <250 μm for the lower and upper lids, respectively, to treated similar depths as electrolysis. The higher proportion of actively growing upper lid follicles explains why upper lid lashes are longer.