The objective of this study was to assess the validity of prevalent and incident self-reported diabetes compared with multiple reference definitions and to assess the reliability (repeatability) of a self-reported diagnosis of diabetes. Data from 10,321 participants in the Atherosclerosis Risk in Communities (ARIC) Study who attended visit 4 (1996-1998) were analyzed. Prevalent self-reported diabetes was compared with reference definitions defined by fasting glucose and medication use obtained at visit 4. Incident self-reported diabetes was assessed during annual follow-up telephone calls and was compared with reference definitions defined by fasting glucose, hemoglobin A1c, and medication use obtained during an in-person visit attended by a subsample of participants (n = 1,738) in 2004-2005. The sensitivity of prevalent self-reported diabetes ranged from 58.5% to 70.8%, and specificity ranged from 95.6% to 96.8%, depending on the reference definition. Similarly, the sensitivity of incident self-reported diabetes ranged from 55.9% to 80.4%, and specificity ranged from 84.5% to 90.6%. Percent positive agreement of self-reported diabetes during 9 years of repeat assessments ranged from 92.7% to 95.4%. Both prevalent self-reported diabetes and incident self-reported diabetes were 84%-97% specific and 55%-80% sensitive as compared with reference definitions using glucose and medication criteria. Self-reported diabetes was >92% reliable over time.