Web-based personal health records (PHRs) have been advocated as a means to improve type 2 diabetes mellitus (DM) care. However, few Web-based systems are linked directly to the electronic medical record (EMR) used by physicians.We randomized 11 primary care practices. Intervention practices received access to a DM-specific PHR that imported clinical and medications data, provided patient-tailored decision support, and enabled the patient to author a "Diabetes Care Plan" for electronic submission to their physician prior to upcoming appointments. Active control practices received a PHR to update and submit family history and health maintenance information. All patients attending these practices were encouraged to sign up for online access.We enrolled 244 patients with DM (37% of the eligible population with registered online access, 4% of the overall population of patients with DM). Study participants were younger (mean age, 56.1 years vs 60.3 years; P < .001) and lived in higher-income neighborhoods (median income, $53,784 vs $49,713; P < .001) but had similar baseline glycemic control compared with nonparticipants. More patients in the intervention arm had their DM treatment regimens adjusted (53% vs 15%; P < .001) compared with active controls. However, there were no significant differences in risk factor control between study arms after 1 year (P = .53).Previsit use of online PHR linked to the EMR increased rates of DM-related medication adjustment. Low rates of online patient account registration and good baseline control among participants limited the intervention's impact on overall risk factor control.clinicaltrials.gov Identifier: NCT00251875.