Unrecognized leukemia cutis.

In a 76-year-old woman with a history of breast cancer, a diffuse erythematous maculonodular skin eruption was thought from biopsy results to be due to metastatic breast carcinoma. Peripheral blood and bone marrow showed no abnormalities. The patient received only tamoxifen for presumed recurrent breast carcinoma. The skin lesions totally resolved over the next nine months, but acute myelomonocytic leukemia subsequently developed. The leukemic blasts in the bone marrow were identical to the malignant cells in the previous skin biopsies. Correct identification of neoplastic skin infiltrations has become increasingly important, since effective therapy is available for many types of neoplasms. Cytochemical or immunohistochemical staining and/or electron microscopy should be considered if the appearance on light microscopy is inconclusive.