Abnormal-looking melanosomes are observed commonly in both intraepidermal melanocytes of cutaneous melanoma and dysplastic melanocytic nevi (DMN). This was investigated by using transmission electron microscopic examination to determine the percentage of abnormal melanosomes among 8267 melanosomes assessed in at least five solitary intraepidermal basal unit melanocytes from each of five specimens of DMN, superficial spreading melanoma (SSM), common acquired nevomelanocytic nevi (NMN), and normal skin (NS) adjacent to DMN. The percentage of abnormal melanosomes in DMN (mean + SD, 44 + 23%) was seven times greater than that in NMN (6 + 7%) and 22 times greater than that in NS (2 + 5% [P less than 0.001, both comparisons]), but only 80% that in SSM (57 + 19% [P less than 0.02]). Melanocyte area, nuclear area, and the ratio of nuclear area to cytoplasmic area did not account for the observed differences. Melanosomal alterations may be a useful marker of atypicality in melanocytic tumors.