A 29-yr old woman developed urticaria pigmentosa which subsequently progressed through systemic mastocytosis to Philadelphia chromosome negative (Ph neg) chronic myelogenous leukemia (CML) with t(8;17). Further cytogenetic evolution occurred at the time of transformation to the aggressive phase of the disease. Unlike Ph-positive CML, chromosome number 9 was not involved, nor was the breakpoint cluster region located at band 22q11. This clearly separates this case from other Ph-negative CML patients who do have involvement of 9q34[1] or the breakpoint cluster region [2, 3]. Since this is the first case of its type to be reported with cytogenetic abnormalities, the clinical relevance of the unique chromosomal rearrangement t(8;17)(pll;q25) in the setting of systemic mastocytosis is unclear. Additional cases need to be reported to determine if this genetic rearrangement is a nonrandom marker of leukemia evolving in a setting of malignant mast cell disease.