OBJECTIVE: To test the hypothesis that patient readiness to change (RTC) predicts future changes in glycemic control in adults with diabetes. RESEARCH DESIGN AND METHODS: We linked survey data with HbA1c data for a stratified random sample of consenting adults with diabetes. Change in HbA1c from baseline to the 1-year follow-up was computed and used as a dependent variable. Linear regression models assessed RTC and other patient variables as predictors of HbA1c change. RESULTS: Among 617 patients with baseline HbA1c > or = 7% and complete data for analysis, RTC predicted subsequent improvement in HbA1c for those with higher physical functioning (interaction t = -2.45, P < 0.05). Other factors that predicted HbA1c improvement in multivariate linear regression models included higher self-reported medication adherence (t = -4.41, P < 0.01), higher baseline HbA1c (t = -15.08, P < 0.01), and older age (t = -2.61, P < 0.01). CONCLUSIONS: Diabetes RTC independently predicts change in HbA1c for patients with high but not for patients with low functional health status. Customized use of RTC assessment may have potential to improve care.