INTRODUCTION: Moderate-intensity physical activity is recommended for individuals with diabetes to control glucose and prevent diabetes-related complications. The extent to which a diabetes diagnosis motivates patients to increase physical activity is unclear. This study used data from the Women's Health Initiative Observational Study (baseline data collected from 1993 to 1998) to examine change in physical activity and sedentary behavior in women who reported a diabetes diagnosis compared with women who did not report diabetes during 7 yr of follow-up (up to 2005). METHODS: Participants (n = 84,300) were postmenopausal women who did not report diabetes at baseline (mean +/- SD; age = 63.49 +/- 7.34 yr; body mass index = 26.98 +/- 5.67 kg.m). Linear mixed-model analyses were conducted adjusting for study year, age, race/ethnicity, body mass index, education, family history of diabetes, physical functioning, pain, energy/fatigue, social functioning, depression, number of chronic diseases, and vigorous exercise at age 18 yr. Analyses were completed in August 2012. RESULTS: Participants who reported a diabetes diagnosis during follow-up were more likely to report increasing their total physical activity (P = 0.002), walking (P < 0.001), and number of physical activity episodes (P < 0.001) compared with participants who did not report a diabetes diagnosis. On average, participants reporting a diabetes diagnosis reported increasing their total physical activity by 0.49 MET.h.wk, their walking by 0.033 MET.h.wk, and their number of physical activity episodes by 0.19 MET.h.wk. No differences in reported sedentary behavior change were observed (P = 0.48). CONCLUSIONS: A diabetes diagnosis may prompt patients to increase physical activity. Healthcare professionals should consider how best to capitalize on this opportunity to encourage increased physical activity and maintenance.