PURPOSE: We undertook this study to examine the symptoms, clinical events, and types of health care encounters that preceded the diagnosis of diabetes mellitus in adults, and to examine changes in glycemic control and cardiovascular risk factors in the first year after a diabetes diagnosis. METHODS: We conducted a historical cohort study of patients in a large multispecialty medical group in Minnesota. Among 55,121 adults who were continuously enrolled in the health plan and receiving care at the study medical group from January 1, 1993, to December 31, 1996, we identified 504 who received a new diagnosis of diabetes in 1995 or 1996. Our main outcome measures were the type of symptoms at diagnosis; the clinical circumstances and type of encounter that led to diabetes diagnosis; and changes in glycemic control (assessed by hemoglobin A1c [HbA1c] value), low-density lipoprotein cholesterol level, blood pressure (BP), aspirin use, and body weight in the first year after diagnosis, ascertained from a detailed review of medical records. RESULTS: Almost one third (32.3%) of adults with newly diagnosed diabetes had symptoms of hyperglycemia at initial diagnosis. Compared with patients who did not have hyperglycemia symptoms at diagnosis, those who did were younger and more often male, and had lower comorbidity scores and higher HbA1c values (9.9% vs 8.1%) at diagnosis (P <.01 for each comparison). In the 12 months after diagnosis, the group as a whole had significant improvements (P <.001) in HbA1c values (from 8.8% to 7.1%), systolic blood pressure (137.5 to 133.2 mm Hg), diastolic blood pressure (80.7 to 77.3 mm Hg), weight (207.7 to 201.1 lb), and aspirin use (15.3% to 26.1%). Improvements were seen in all patient subgroups, including those defined by symptoms at diagnosis and by visit type at diagnosis. CONCLUSIONS: Primary care practices may improve detection of undiagnosed diabetes in primary care and improve 1-year outcomes by being vigilant for symptoms of diabetes, by evaluating those at high risk for this disorder, and by instituting appropriate treatments at the time of diagnosis.