BACKGROUND:Existing tools for measuring blood-borne virus (BBV) and sexually transmitted infection (STI) transmission risk behaviours in substance use interventions have limited capacity to assess risk behaviours across varied social, cultural and epidemiological contexts; have not evolved alongside HIV treatment and prevention innovations; or accounted for sexual contexts of drug use including among a range of lesbian, gay, bisexual, transgender, intersex and queer (LGBTIQ) sub-communities. The Substance Use and Sex Index (SUSI) is a new brief, simple tool being developed to assess change in HIV and STI risk behaviours for substance use treatment studies. METHODS:A 26-item questionnaire was piloted online among community volunteers (n = 199). Concurrent and predictive validity were assessed against risk-taking (RT-18) and STI testing items (Gay Community Periodic Surveys). RESULTS:The developed scale comprised nine items measuring: condomless penile (anal or vaginal) sex, unprotected oral sex, shared toy use, bloodplay, chemsex (consumption of drugs for the facilitation of sex), trading sex for drugs, being 'too out of it' to protect self, injecting risk and group sex. Factor-analytic approaches demonstrated that items met good fit criteria for a single scale. Significant, moderate magnitude, positive relationships were identified between total SUSI score and both RT-18 risk-taking and recent STI testing. Qualitative feedback underscored the importance of culturally-embedded question formulation. CONCLUSION:The results support the conceptual basis for the instrument, highlighting the need for further scale content refinement to validate the tool and examine sensitivity to change. SUSI is a step towards improving outcome measurement of HIV/BBV/STI transmission risk behaviours in substance use treatment studies with greater inclusiveness of experiences across different population groups.