A 68-year-old woman with acute myelomonocytic leukemia, who was treated annually for 21 consecutive years by "therapeutic" low-dose radon gas radiation because of spondyloarthritis, is described. The karyotype of the malignant clone was 45,XX, -17, -18,del(5)(q15q33), +t(17;18)(q11.2q23). In 45% of the metaphases, the modal number was between hyperdiploid to near tetraploid. Double minute chromosomes were demonstrated in 60% of the cells. These chromosomal aberrations are suggestive of mutagen-related leukemia.