To examine the effect of patient-directed electronic messages on health care proxy (HCP) use.Randomized control trial using an electronic message intervention to encourage patients to complete an HCP document.General medical practice at a large tertiary care teaching institution.Nine hundred twelve patients aged older than 50.We reviewed online medical records (OMRs) to assess for discussion and documentation of HCPs and to collect information on patient characteristics. We surveyed participants to determine knowledge, discussion, and completion of HCPs.Four hundred thirty participants were randomized to the intervention group and 482 to the control group. Only 1 HCP discussion (intervention group) and only 10 new HCPs (4 in intervention group versus 6 in control group, p = 0.649) were documented in the OMR. Among the 444 survey responders, 205 (46%) reported having an HCP, but only 74 (36%) of these had discussed the HCP with their doctors and only 9 (4%) had a documented HCP in the OMR. Patients in the intervention group were more likely to report knowledge of HCPs (adjusted risk ratio [RR] 1.07; 95% confidence interval [CI], 1.01-1.14) and having a plan to complete one in the future (adjusted RR 1.19; 95% CI, 1.05-1.36).This patient-directed intervention did not increase patient completion of an HCP but was associated with greater knowledge of an HCP and planning to complete one.