Examination of the skin or hair with Wood's lamp has long been used to aid in the clinical diagnosis of some cutaneous disorders. A new observation reported here illustrates that Wood's light can be used to determine the depth of melanin pigmentation in the skin: contrast in epidermal pigmentation is increased while contrast in dermal pigmentation is decreased under Wood's lamp illumination compared to ambient visible light. The principles underlying this phenomenon are discussed. Use of the Wood's lamp to localize abnormal melanin pigmentation in the skin can be a guide to clinical diagnosis of hypermelanosis.