BACKGROUND: The number of people living with multiple chronic conditions is increasing, but we know little about the impact of multimorbidity on life expectancy. OBJECTIVE: We analyze life expectancy in Medicare beneficiaries by number of chronic conditions. RESEARCH DESIGN: A retrospective cohort study using single-decrement period life tables. SUBJECTS: Medicare fee-for-service beneficiaries (N=1,372,272) aged 67 and older as of January 1, 2008. MEASURES: Our primary outcome measure is life expectancy. We categorize study subjects by sex, race, selected chronic conditions (heart disease, cancer, chronic obstructive pulmonary disease, stroke, and Alzheimer disease), and number of comorbid conditions. Comorbidity was measured as a count of conditions collected by Chronic Conditions Warehouse and the Charlson Comorbidity Index. RESULTS: Life expectancy decreases with each additional chronic condition. A 67-year-old individual with no chronic conditions will live on average 22.6 additional years. A 67-year-old individual with 5 chronic conditions and ≥10 chronic conditions will live 7.7 fewer years and 17.6 fewer years, respectively. The average marginal decline in life expectancy is 1.8 years with each additional chronic condition-ranging from 0.4 fewer years with the first condition to 2.6 fewer years with the sixth condition. These results are consistent by sex and race. We observe differences in life expectancy by selected conditions at 67, but these differences diminish with age and increasing numbers of comorbid conditions. CONCLUSIONS: Social Security and Medicare actuaries should account for the growing number of beneficiaries with multiple chronic conditions when determining population projections and trust fund solvency.