© 2014 by the American College of Sports Medicine. Purpose The objective of this study is to report methodological details and feasibility of conducting an accelerometer ancillary study in a large US cohort being followed for stroke and cognitive decline. Methods Reasons for Geographic and Racial Differences in Stroke is a national population-based study of 30,239 blacks and whites, age ≥45 yr, enrolled January 2003 to October 2007. Baseline evaluations were conducted through computer-assisted telephone interview and an in-home visit. Participants are followed by computer-assisted telephone interview every 6 months. Starting with May 2009 follow-up, contingent on accelerometer availability, participants were invited to wear an accelerometer for 7 d. Device inventory was 1150. Accelerometer, instructions, log sheet, and stamped addressed return envelope were mailed to consenting participants. Postcard acknowledgement and reminders and two calls or less were made to encourage compliance. Results Between May 2009 and January 2013, 20,076 were invited to participate; 12,146 (60.5%) consented. Participation rates by race-sex groups were similar: black women, 58.6%; black men, 59.6%; white women, 62.3%; and white men, 60.5%. The mean age of the 12,146 participants to whom devices were shipped was 63.5 ± 8.7 yr. Return rate was 92%. Of 11,174 returned, 1187 were not worn and 14 had device malfunction, and of 9973 with data, 8096 (81.2%) provided usable data, defined as ≥4 d of 10+ h of wear time, ranging from 74.4% among black women to 85.2% among white men. Conclusions Using mail and telephone methods, it is feasible to obtain objective measures of physical activity from a sizeable proportion of a national cohort of adults, with similar participation rates among blacks and whites. Linked with the clinical health information collected through follow-up, these data will allow future analyses on the association between objectively measured sedentary time, physical activity, and health outcomes.