Background Prior studies have described patient characteristics, treatments, and outcomes for older adults with myelodysplastic syndromes, but most have failed to distinguish chronic myelomonocytic leukemia. Recognizing potentially important differences between the diseases, we undertook a population-based comparison of baseline characteristics, treatments, and outcomes between older adults with chronic myelomonocytic leukemia and myelodysplastic syndromes. Design and Methods Patient data were obtained from Surveillance Epidemiology and End Results registry data from 2001-2005, linked to Medicare claims. Baseline characteristics, treatment (red blood cell transfusions, hematopoietic growth factors, hypomethylating agents, chemotherapy or transplantation) , progression to acute myeloid leukemia, and overall survival were compared using bivariate techniques. Multivariate logistic regression estimated differences in treatments received. Cox Proportional Hazard models estimated the effects of chronic myelomonocytic leukemia relative to myelodysplastic syndromes on progression free survival. Results A larger proportion of chronic myelomonocytic leukemia (n=792), compared to myelodysplastic syndrome patients (n=7,385), failed to receive any treatment (25% vs. 15%; p< 0.0001), or only received red blood cell transfusions (19.8% vs. 16.7%; p= 0.037). A larger percentage of chronic myelomonocytic leukemia patients progressed to acute myeloid leukemia (42.6% vs. 15.5%; p< 0.000.1), with shorter time to progression. Chronic myelomonocytic leukemia patients had an inferior median survival (13.3 vs. 23.3 months; p< 0.0001) and 3-year survival (19% vs. 36%; p< 0.0001). Adjusted estimates, controlling for baseline characteristics and selected treatments, indicate that chronic myelomonocytic leukemia was associated with an increased risk of acute myeloid leukemia progression or death (HR 2.22, p<0.0001), compared to myelodysplastic syndromes. Conclusions Chronic myelomonocytic leukemia is less frequently treated in older adults and is associated with worse outcomes, even after controlling for baseline patient characteristics and selected treatments. Our data suggest the need for continued evaluation of the biologic differences between these diseases and clinical trials targeting chronic myelomonocytic leukemia.