INTRODUCTION AND AIMS: We investigated the Australian public's understandings of addiction to alcohol and heroin and the factors predicting support for the idea that these types of addiction are 'diseases' and specifically 'brain diseases'. DESIGN AND METHODS: Data were collected as part of the 2012 Queensland Social Survey, a computer-assisted telephone interview of 1263 residents of Queensland, Australia. Participants were presented with scenarios of two addicted persons, one who was addicted to heroin and the other addicted to alcohol. Participants were asked a series of questions about different definitions and causes of addiction for both characters. RESULTS: Over half of the respondents thought that addiction is a disease (alcohol: 67%, heroin: 53%), but fewer (alcohol: 34%, heroin: 33%) believed that addiction is a brain disease. Belief that addiction has biological causes predicted agreement that addiction is a disease [alcohol: odds ratio (OR) = 3.05 (2.15-4.31), heroin: OR = 3.99 (2.82-5.65)] and a brain disease [alcohol: OR = 4.97 (3.42-7.22), heroin: OR = 14.12 (9.23-21.61)]. Women were more likely than men to agree that addiction is a disease [alcohol: OR = 1.79 (1.35-2.38), heroin: OR = 1.40 (1.09-1.81)] as were those 35 years of age and older [alcohol: OR = 2.25 (1.50-3.40), heroin: OR = 1.50 (1.01-2.24)]. DISCUSSION AND CONCLUSIONS: There is more public support for the idea that addiction is a 'disease' than for the more specific claim that it is a 'brain disease'. Support for a biological aetiology of addiction predicted higher levels of agreement with both disease concepts.