We report a case of a 27-year-old male showing Becker's nevus on his trunk and lower extremities. It represented symmetrically bilateral and extensive(about 50% of body surface) involvement. In addition to the usual clinical findings of Becker's nevus, grouped small darker spots were noted on the lesion of the buttock featuring agminated lentigine. Histologic findings of the brownish pigmented lesion consisted of mild acanthosis, hyperkeratosis, elongation of rete ridges, increase of basal layer pigmentation, and numerous terminal hair follicles. A dark macule in agminated lesions on the buttock showed more dense basal layer pigmentation and more regular elongation and fusion of rete ridges consistent with the histologic findings of lentigo simplex. This report describes a rare case of a giant Becker's nevus involving 50% of body surface in a bilateral and symmetrical distribution with associated agminated lentigine. (Korean J Dermatol 2001; 39(4): 478~480)