OBJECTIVE: The study's purpose was to describe the cumulative duration of 19 chronic conditions among Medicare fee-for-service (FFS) beneficiaries and examine variation in total expenditures explained by cumulative duration and condition counts. DESIGN, SETTING, STUDY DESIGN, AND DATA EXTRACTION: In a retrospective cohort of FFS beneficiaries age ≥68, 2015 Medicare enrollment and claims data (N = 20 124 230) were used to identify the presence or absence of 19 diagnosed chronic conditions, and to construct MCC categories (0-1, 2-3, 4-5, 6+) and cumulative duration of each of 19 conditions from the date of first possible occurrence in claims (1/1/1999) to the end of follow-up (date of death or 12/31/2015). Total Medicare expenditures were estimated using linear models adjusted for demographic characteristics. PRINCIPAL FINDINGS: Multimorbidity was common (71.7 percent with 2+ conditions). The mean cumulative duration of all 19 conditions was 23.6 person-years, which varied greatly by age and number of conditions. Condition counts were more predictive of Medicare expenditures than cumulative duration (R-squared for continuous measures = 0.461 vs 0.272; R-squared for quartiles = 0.408 vs 0.266). CONCLUSIONS: The cumulative duration of chronic conditions varied widely for Medicare beneficiaries, especially for those with 6+ conditions, but was less predictive of total expenditures than condition counts.