Pigmented carcinoid tumour of the thymus.

An asymptomatic thymic tumour, classified as a carcinoid tumour on the basis of its light and electron-microscopic appearances, is described. This tumour was atypical in having foci of melanin pigmentation, the pigment being contained in dendritic melanocytes and melanophages. We suggest that the pigmented cells are not neoplastic and represent latent thymic melanoblasts stimulated in some way by the carcinoid tumour. Alternative explanations are discussed.