Abstract Objective To describe the prevalence of multimorbidity involving 20 selected chronic conditions in a geographically defined US population, emphasizing age, sex, and racial/ethnic differences. Patients and Methods Using the Rochester Epidemiology Project records linkage system, we identified all residents of Olmsted County, Minnesota, on April 1, 2010, and electronically extracted the International Classification of Diseases, Ninth Revision codes associated with all health care visits made between April 1, 2005, and March 31, 2010 (5-year capture frame). Using these codes, we defined the 20 common chronic conditions recommended by the US Department of Health and Human Services. We counted only persons who received at least 2 codes for a given condition separated by more than 30 days, and we calculated the age-, sex-, and race/ethnicity-specific prevalence of multimorbidity. Results Of the 138,858 study participants, 52.4% were women (n=72,732) and 38.9% had 1 or more conditions (n=54,012), 22.6% had 2 or more conditions (n=31,444), and 4.9% had 5 or more conditions (n=6853). The prevalence of multimorbidity (≥2 conditions) increased steeply with older age and reached 77.3% at 65 years and older. However, the absolute number of people affected by multimorbidity was higher in those younger than 65 years. Although the prevalence of multimorbidity was similar in men and women overall, the most common dyads and triads of conditions varied by sex. Compared with white persons, the prevalence of multimorbidity was slightly higher in black persons and slightly lower in Asian persons. Conclusion Multimorbidity is common in the general population; it increases steeply with older age, has different patterns in men and women, and varies by race/ethnicity.