OBJECTIVE: When patients miss scheduled medical appointments, continuity and effectiveness of healthcare delivery is reduced, appropriate monitoring of health status lapses, and the cost of health services increases. We evaluated the relationship between missed appointments and glycemic control (glycosylated hemoglobin or HbA1c) in a large, managed care population of diabetic patients. RESEARCH DESIGN AND METHODS: Missed appointment rate was related cross-sectionally to glycemic control among 84,040 members of the Kaiser Permanente Northern California Diabetes Registry during 2000. Adjusted least-square mean estimates of HbA1c were derived by level of appointment keeping (none missed, 1-30% missed, and >30% missed appointments for the calendar year) stratified by diabetes therapy. RESULTS: Twelve percent of the subjects missed more than 30% of scheduled appointments during 2000. Greater rates of missed appointments were associated with significantly poorer glycemic control after adjusting for demographic factors (age, sex), clinical status, and health care utilization. The adjusted mean HbA1c among members who missed >30% of scheduled appointments was 0.70 to 0.79 points higher (P <0.0001) relative to those attending all appointments. Patients who missed more than 30% of their appointments were less likely to practice daily self-monitoring of blood glucose and to have poor oral medication refill adherence. CONCLUSION: Patients who underuse care lack recorded information needed to determine level of risk. Frequently missed appointments were associated with poorer glycemic control and suboptimal diabetes self-management practice, are readily ascertained in clinical settings, and therefore could have clinical utility as a risk-stratifying criterion indicating the need for targeted case management.