The identification of malignant melanoma a visceral organ of nonepidermal o
rigin is not an uncommon occurence. Frequently, these cases are solitary me
tastases that present years after a thin epidermal melanoma has been diagno
sed (and sometimes forgotten). However, primary visceral melanomas have bee
n reported that have not been preceded by an epidermal lesion. We describe
herein a unique case of melanoma presenting as a primary intrathymic tumor.
The patient had no previous history of epidermal melanoma, and extensive w
orkup did not reveal evidence for an alternative primary site. The tumor ex
hibited histologic features characteristic of melanoma including an abundan
ce of large pleomorphic cells with eosinophilic cytoplasm, prominent nucleo
li, and occasional intranuclear inclusions. Tumor cells stained for HMB-45
and S100 protein and ultrastructure analysis revealed stage II and stage II
I melanosomes. The patient remained free of disease until intrathoracic rec
urrence was detected on a computed tomographic scan 14 months later. The la
ck of clinical history and physical findings of melanoma at presentation, t
he intrathymic location of the tumor, and the pattern of recurrence suggest
that this case likely represents primary thymic melanoma, a previously unr
eported entity.