Introduction and Aims: Staff interactions with their clients are an important factor in the quality of care that is provided to people in drug treatment. Yet there is very little research that addresses staff attitudes or clients' perceptions of discrimination and prejudice by staff with regard to treatment outcomes. This research aimed to assess whether perceptions of discrimination by staff predict drug treatment completion. Design and Methods: The study used a mixed methods approach. Ninety-two clients in residential rehabilitation facilities in Sydney were administered a series of quantitative measures assessing drug history, severity of drug use, treatment history, perceptions of staff discrimination and treatment motivation. Clients were followed up regularly until an outcome (dropout or completion) was obtained for the full sample. Results: Perceptions of discrimination were a significant predictor of treatment completion, with greater perceived discrimination associated with increased dropout. Qualitative interviews with 13 clients and eight health-care workers from these treatment services were then conducted to gain insight into how perceived discrimination may impact on treatment experiences. Clients and staff discussed how they would address the issue of perceived discrimination during the current treatment experience. Discussion and Conclusions: Adopting a mixed methods approach facilitated exploration of the impact of perceived discrimination on treatment from both clients' and health-care workers' perspectives. This methodology may also enhance interpretation and utilisation of these findings in drug treatment.