Objective: To investigate light microscopic and ultrastructural changes in bimatoprost-induced skin hyperpigmentation. Methods: Eyelid biopsy specimens from bimatoprost-treated patients and matched controls were examined by light microscopy and transmission electron microscopy. Using an image analyzer, melanin granules were counted on Fontana-Masson-stained sections, and melanosomes were counted on electron micrographs. Immunohistochemical analysis was performed with antibodies against S100 and CD3. Positively labeled cells were counted. Results: By light microscopy, a marked increase in the number of melanin granules was noted in the bimatoprost-treated specimens. Electron microscopy demonstrated dermal melanocytes with prominent rough endoplasmic reticulum and abundant normal-sized melanosomes in different stages of maturation as compared with control specimens. Furthermore, the keratinocytes of the bimatoprosttreated specimens showed abundant mature melanosomeswhen compared with controls. Also of note, atypical melanocytes were absent in both specimens. The S100-positive melanocytes were comparable in bimatoprost-treated and control specimens. Few CD3-and CD68-positi ve cells in the bimatoprost-treated specimens were noted in both groups. Conclu sion: Bimatoprost-induced periocular hyperpigmentation is caused by increa sed melanogenesis. There was no evidence of melanocyte proliferation or prostagl andin-induced inflammation in the specimens that were examined. Objective: To investigate light microscopic and ultrastructural changes in bimatoprost-induced skin hyperpigmentation. Methods: Eyelid biopsy specimens from bimatoprost-treated patients and matched controls were examined by light microscopy and transmission electron microscopy. Using an image analyzer, melanin granules were counted on Fontana -Masson-stained sections, and melanosomes were counted on electron micrographs. Immunohistochemical analysis was performed with antibodies against S100 and CD3. Positively labeled cells were counted in the results of By light microscopy, a marked increase in the number of melanin granules was noted in the bimatoprost-treated specimens. Electron microscopy of dermal melanocytes with prominent rough endoplasmic reticulum and abundant normal-sized melanosomes in different stages of maturation as compared with control specimens. Furthermore, the keratinocytes of the bimatoprostreated specimens showed abundant mature melanosomes were compared with con The S100-positive melanocytes were comparable in bimatoprost-treated and control specimens. Few CD3-and CD68-positi ve cells in the bimatoprost-treated specimens were noted in both groups. Conclu sion: Bimatoprost-induced periocular hyperpigmentation is caused by increa sed melanogenesis. There was no evidence of melanocyte proliferation or prostagl and in-induced inflammation in the specimens that were examined.