Develops from superficial spreading melanoma. Melanocytic infiltration reaches deep into the corium and vertical growth starts to prevail. Groups of melanocytes of variable size are formed, contributing to the asymmetry of the lesion. The melanocytes within these groups are usually large, epithelioid, low melanin production, with prominent atypia, nucleoli and mitotic activity.

In time large nodularity reaching into the deep dermis or subcutis is formed. The superficial part of the tumor continues to spread laterally and is wider than the tumor; epidermal scatter is present.