Malignant blue naevus is a distinct but rarely documented variant of malignant melanoma, and we describe the triple recurrence of a suprapatellar cellular blue naevus over 12 years in a middle-aged woman. Staging investigations revealed a distant subcutaneous metastasis of the right thigh. Immunohistochemistry of the primary lesion and all recurrences showed S-100, HMB-45, NK1/C-3 and Ki-67 positive cells. However, non-malignant cellular blue naevi from five consecutive other patients were all Ki-67 negative. The change from negative to positive Ki-67 responsivity may therefore be a valuable marker of malignant and metastatic potential in early cellular blue naevi.