Results: One third of the melanomas grew 0.5 mm per month or more. The median monthly growth rate was 0.12 mm for superficial spreading melanomas, 0.13 mm for lentigo maligna melanomas, and 0.49 mm for nodular melanomas. Rapid tumor growth was associated with tumor thickness (≤1 mm, geometric mean ratio [GMR] = 1.0; 1.01-4 mm, GMR = 3.9; and >4 mm, GMR = 12.1) and mitotic rate (<1/mm2, GMR = 1.0; 1-4/mm2, GMR = 2.9; 5-10/mm2, GMR = 6.1; and >10/mm2 GMR = 9.7). Rapid tumor growth occurred more often in males (GMR = 1.7), elderly individuals (≥70 years old, GMR = 2.8), and patients with fewer melanocytic nevi (n<50, GMR = 2.0) and fewer freckles (GMR = 2.5). Rapidly growing melanomas were more often symmetrical (GMR = 2.5), elevated (GMR = 1.4), amelanotic (GMR = 1.7), regular in border (GMR = 2.5), and symptomatic (GMR = 1.7).