Background: Law purports to regulate end-of-life care but its role in decision-making by doctors is not clear. This paper, which is part of a three-year study into the role of law in medical practice at the end of life, investigates whether law affects doctors' decision-making. In particular, it considers whether the fact that the law differs across Australia's three largest states - New South Wales (NSW), Victoria and Queensland - leads to doctors making different decisions about withholding and withdrawing life-sustaining treatment from adults who lack capacity. Methods: A cross-sectional postal survey of the seven specialties most likely to be involved in end-of-life care in the acute setting was conducted between 18 July 2012 and 31 January 2013. The sample comprised all medical specialists in emergency medicine, geriatric medicine, intensive care, medical oncology, palliative medicine, renal medicine and respiratory medicine on the AMPCo Direct database in those three Australian states. The survey measured medical specialists' level of legal compliance, and reasons for their decisions, concerning the withholding or withdrawal of life-sustaining treatment. Multivariable logistic regression was used to examine predictors of legal compliance. Linear regression was used to examine associations between the decision about life-sustaining treatment and the relevance of factors involved in making these decisions, as well as state differences in these associations. Results: Response rate was 32% (867/2702). A majority of respondents in each state said that they would provide treatment in a hypothetical scenario, despite an advance directive refusing it: 72% in NSW and Queensland; 63% in Victoria. After applying differences in state law, 72% of Queensland doctors answered in accordance with local law, compared with 37% in Victoria and 28% in NSW (p < 0.001). Doctors reported broadly the same decision-making approach despite differences in local law. Conclusions: Law appears to play a limited role in medical decision-making at the end of life with doctors prioritising patient-related clinical and ethical considerations. Different legal frameworks in the three states examined did not lead to different decisions about providing treatment. More education is needed about law and its role in this area, particularly where law is inconsistent with traditional practice.