Distinctive melanocytic moles are described in 37 patients from six melanoma families. Among the family members examined by the authors, 15 of 17 patients with melanoma and 22 of 41 nonmelanoma relatives had the unique moles. The clinical and histological features of these moles have been designated the "B-K mole syndrome." The clinical features of the syndrome include the presence of less than 10 to greater than 100 moles prominent of the upper trunk and extremities, and variability of mole size (5 mm to 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 two B-K moles into malignant melanomas was documented photographically.