Colorectal cancer (CRC) has the second highest cancer mortality rate in Australia. The Australian National Bowel Cancer Screening Program (NBCSP) aims to increase early detection of CRC by offering free Faecal Occult Blood Testing (FOBT), although uptake is low for culturally and linguistically diverse (CALD) groups.To present data on trust and mistrust in the NBCSP by population groups with low uptake and thus to highlight areas in need of policy change.A qualitative study was undertaken in South Australia, involving interviews with 94 people from four CALD groups: Greek, Iranian, Anglo-Australian, and Indigenous peoples.Our study highlights the complexities of institutional trust, which involves considerations of trust at interpersonal, local and national levels. In addition, trust and mistrust was found in more abstract systems such as the medical knowledge of doctors to diagnose or treat cancer or the scientific procedures in laboratories to test the FOBTs. The object of institutional (mis)trust differed between cultural groups - Anglo-Australian and Iranian groups indicated a high level of trust in the government, whereas Indigenous participants were much less trusting.The level and nature of trust in the screening process varied between the CALD groups. Addressing program misconceptions, clarifying the FOBT capabilities and involving medical services in collecting and transporting the samples may increase trust in the NBCSP. However, broader and more enduring mistrust in services and institutions may need to be dealt with in order to increase trust and participation.