International Journal of Epidemiology, 2015, Vol. 44, Supplement 1 i39 Risky Behaviors: Free Papers Abstract #: 2474 If you are not counted, you don’t count: Estimating the number of African-American Men who have Sex with Men in San Francisco. P. D. Wesson, BA 1 , M. S. Handcock, PhD 2 , W. McFarland, PhD 3,4 and H. F. Raymond, DrPH 3,4 University of California, Berkeley, Berkeley, CA, USA, 2 University of California, Los Angeles, Los Angeles, CA, USA, 3 University of California, San Francisco, San Francisco, CA, USA, 4 San Francisco Department of Public Health, San Francisco, CA, USA Downloaded from http://ije.oxfordjournals.org/ by guest on October 25, 2016 INTRODUCTION: The HIV epidemic has disproportionately affected African-American (AA) men who have sex with men (MSM). Resource allocation for programs targeting key populations such as AA MSM require reliable estimates of their numbers. Current population size estimation (PSE) methods rely on assump- tions that are difficult to meet, potentially producing large biases. METHODS: We applied a new method to estimate the number of AA MSM in San Francisco (SF) using a respondent-driven sampling (RDS) survey. The approach uses respondents’ self-reported network size (i.e. the number of other AA MSM known to them in SF), along with a specification of the prior knowledge about the population size, to model the total size of the target population based on their proba- bility of recruitment. A Bayesian approach is used to quantify the amount of information on population size available in the survey. The plausibility of the resulting estimate was corroborated against an inde- pendent estimate of all MSM in SF and HIV case reporting data. RESULTS: 259 AA MSM were recruited in the RDS survey. The method calculated a median of 6,230 AA MSM living in SF (95% CI 1,984–35,545). The prevalence of diagnosed HIV in the survey (17.3%) projects 1,078 known HIV cases among AA MSM, compa- rable to the 1,170 actually reported to city’s surveillance system. A previous independent survey estimating 66,487 total MSM in SF, 6.1% AA, estimates 4055 AA MSM. CONCLUSIONS: The new method produced a robust, plausible, and consistent population size estimate for AA MSM. Given that RDS surveys are frequently done in many hidden populations world- wide, the method provides a simple, appealing tool to rapidly pro- duce estimates of the size of high risk populations – a fundamental public health measure that has been scarce for much of the HIV epi- demic. It is a useful complement to existing methods, especially when only RDS data are available.