Distinctive melanocytic moles are described in 37 patients from 6 melanoma families. Among the family members examined, 15 of 17 patients with melanoma and 22 of 41 nonmelanoma relatives had the unique moles. The clinical and histological features of these moles were designated the B-K mole syndrome. The clinical features of the syndrome include the presence of < 10 to > 100 moles prominent on the upper trunk and extremities, and variability of mole size (5-15 mm), outline and color combination. Histologically, B-K moles show atypical melanocytic hyperplasia, lymphocytic infiltration, delicate fibroplasia and new blood vessels that occur within a compound nevus or de novo. The transformation of 2 B-K moles into malignant melanomas was documented photographically.