Pityrosporum and seborrheic keratosis: an association.

Seborrheic keratoses (SK) are one of the most common benign tumors of the skin. Studies have suggested that human papillomavirus or a benign clonal proliferation of epidermal cells is involved in the pathogenesis of some SK's, however, this issue remains to be resolved. Pityrosporum ovale has been implicated in the pathogenesis of seborrheic dermatitis, however, its potential role in association with SK's has not been evaluated as evidenced by the fact that there are no publications in the English-language literature regarding this issue. We observed a common occurrence of Pityrosporum organisms in the stratum corneum of SK on routine histopathologic sections and decided to investigate whether there was an association between these two common entities. We found that in a majority of the SK studied Pityrosporum was present, which was significantly different from controls (verruca vulgaris and basal cell carcinoma).