Porokeratosis of Mibelli: benzylhydrochlorothiazide-induced new lesions accompanied by eosinophilic spongiosis.

We observed the course of development of porokeratosis stimulated by benzylhydrochlorothiazide (BHCTh) in normal-appearing skin of a patient with long-standing stable porokeratosis of Mibelli. A 72-year-old Japanese man had had porokeratosis of Mibelli for more than 50 years. During administration of BHCTh for 1 year because of his hypertension, a lichenoid drug eruption developed over the lesions of porokeratosis on the flexor aspects of his legs. Readministration of BHCTh by another physician for 6 months resulted in the occurrence of a similar drug eruption that converted into typical skin lesions of porokeratosis 8 weeks later. Serial microscopic examination suggested that BHCTh administration resulted in eosinophilic spongiosis and cornoid lamella formation, which developed into epidermal changes characteristic of porokeratosis.