A 59-year-old white man with a history of Sydenham's chorea received a mitral valve prosthesis in 1962. He sustained an anterolateral myocardial infarction in 1983. In 1984, he received a heart transplant. To prevent heart rejection, he was initially treated with cyclosporine 12 mg/kg/day and prednisone 90 mg b.i.d. In 1987, azathioprine 100 mg daily was added. In 1989, at the time of our evaluation, his medications included cyclosporine 80 mg b.i.d., prednisone 10 mg b.i.d., and azathioprine 75 mg/day. Since his heart transplant surgery he had not taken any thiazide medication. The patient noted a lesion on his right thigh; the lesion appeared in 1986, 2 years after his heart transplant. On examination in 1989, the lesion was a 2 cm wide annular plaque with a shiny atrophic center and raised border. Both the clinical appearance and pathology were consistent with a diagnosis of porokeratosis of Mibelli. No family history of porokeratosis was elicited.