PROJECT SUMMARY Sexually transmitted infections (STIs) continue to be a major public health problem afflicting millions of people worldwide. Sexual and gender minorities (SGM), including men who have sex with men (MSM) and transgender women, bear a high burden of STIs, while reporting higher rates of risk factors, economic marginalization, social exclusion, and stigmatization which hinders their access to preventive and therapeutic options for STIs. Little information is available in Colombia regarding the prevalence of STIs in SGMs or the risk factors linked to acquiring and transmitting STIs. The current proposal incorporates the use of Respondent Driven Sampling to recruit a group of transgender women and MSM for STI testing and risk assessment in these often hard to reach populations. This project will use RDS to obtain data to assess the unmet need for prevention, diagnosis, and treatment of STIs in MSM and other SGM populations. It will also facilitate enrollment of transgender women into our U-19 syphilis project. The central overarching hypothesis of our study is that in MSM and transgender women who have sex with other men in Cali, Colombia there is a high, unrecognized prevalence of syphilis, gonorrhea, chlamydia, HPV, and viral hepatitis. To test our hypothesis, we propose to use Respondent Driven Sampling to estimate the prevalence of STIs in MSM and transgender women who have sex with men, in Cali, Colombia. In addition, we will quantify metrics of sexual health, knowledge, attitudes and practices of these populations. It is our contention that this approach will undercover the hidden burden and unmet sexual health needs of these populations and will allow policy makers, health care providers and the local community to act upon this information through intervention planning.