Very late metastasis (27 years) of cutaneous malignant melanoma arising in a halo giant congenital nevus.

We report a case of primary cutaneous melanoma with the incidental finding of a lung metastasis 27 years following the original diagnosis. The case is exceptional in that it is a late metastasis of a melanoma that arose in association with a halo giant congenital nevus. The original tumor was a large dermal/subcutaneous nodule composed of very well-differentiated cells reminiscent of type B nevomelanocytes. The metastasis displayed similar histology. This case emphasizes the unpredictable behavior of malignant melanoma. In cases with 'unusual' histology such as this one, the usual prognostic parameters are less helpful in predicting survival. Melanoma should be included in the differential diagnosis of an undiagnosed lesion suspicious for metastasis even if the primary was removed in the remote past.