Context. Primary care physicians are well placed to identify patients in need of advance care planning (ACP) and initiate ACP in advance of an acute situation. Objectives. This study aimed to understand Australian general practitioner (GP) clinical decision making relating to a patient's "need for ACP" and the likelihood of initiating ACP. Methods. An experimental vignette study pseudorandomly manipulated factors thought to influence decision making regarding ACP. Patient-level factors included gender, age, type of disease, medical severity, openness to ACP, doctor-patient relationship, and family support. An accompanying demographic survey assessed health professional-level factors, including gender, years of experience, place of training, place of practice, caseload of patients with ACP, direct personal experience in ACP, and self-reported attitudes toward ACP. Seventy GPs were recruited, and each completed six unique vignettes, providing ratings of patient need for ACP, importance of initiating ACP in the coming months, and likelihood of initiating ACP at the next consultation. Results. Older patients, with malignant or cardiovascular disease, severe clinical presentations, good doctor-patient relationship, female gender, and poor family support were more likely to receive prompt ACP. Positive GP attitudes toward ACP were associated with greater likelihood of initiating ACP promptly. Conclusion. Patients with presentations suggesting higher mortality risk were identified as being in need of ACP; however, the likelihood of initiating ACP was sensitive to GP attitudes and psychosocial aspects of the doctor-patient interaction. Training materials aimed at encouraging GP involvement in ACP should target attitudes toward ACP and communication skills, rather than focusing solely on prognostic risk. (c) 2016 American Academy of Hospice and Palliative Medicine. Published by Elsevier Inc. All rights reserved.