ABSTRACT Forced displacement is on the rise, particularly in areas with high HIV prevalence. Disruption of social networks following forced displacement can amplify the risks of substance use disorders (SUDs) and enable HIV transmission. HIV transmission within and between forcibly displaced and local communities in situations of forced displacement can be facilitated by delayed HIV diagnosis and less viral suppression due to financial, logistical and administrative barriers in access to HIV care encountered by displaced people. Forcibly displaced men who have sex with men (fdMSM) can experience additional barriers due to stigmatization of their sexual orientation (MSM stigma), particularly in countries with high prevalence of MSM stigma such as Ukraine. Since Ukraine has one of the largest HIV epidemics in Europe and is experiencing a war that resulted in internal displacement of an estimated 5.9 million Ukrainians, fdMSM in Ukraine face unique challenges with respect to access to HIV care. We propose a study of 1,200 MSM (600 local and 600 fdMSM) in Ukraine: 800 MSM (400 local and 400 fdMSM) in Kyiv, the capital of Ukraine, and 400 (200 local and 200 fdMSM) in Lviv, a city in Western Ukraine; both cities are hosting large numbers of internally displaced people. Modified respondent-driven sampling will be used to recruit participants and collect social network and behavioral data. Blood samples will be collected from participants living with HIV for viral load testing, HIV genetic sequencing and subsequent phylogenetic analysis. All MSM living with HIV will be invited for follow-up interviews after 6- and 12-months postbaseline recruitment. Our overarching hypothesis is that fdMSM will have higher prevalence of SUDs, HIV, risk-related sexual behaviors, and be more likely to appear in recent HIV transmission clusters compared to local MSM, and that fdMSM in Lviv will have more stigmatizing social network ties that originated after displacement compared to fdMSM in Kyiv. We will address the following Specific Aims: 1) Determine the prevalence of HIV, SUDs, risk-related sexual behaviors, MSM stigma, and HIV testing among local and fdMSM in Kyiv and Lviv, Ukraine; 2) Characterize changes in social network composition since February 2022 and the impact of these changes on SUDs, MSM stigma, HIV risk behaviors, HIV testing, and viral suppression (among MSM living with HIV) in both local and fdMSM; 3) Estimate the impact of MSM stigma and social support disruption on HIV transmission dynamics between and within local MSM and fdMSM using phylodynamic analysis.