Denver Public Health implements the National HIV Behavioral Surveillance System (NHBS), a cyclical survey of populations at increased risk for HIV. We evaluated the implementation of NHBS among Denver men who have sex with men (MSM), considering the system's simplicity, data quality, representativeness, and sensitivity to trends. We found that the time required for implementation and the complexity of data management and analysis are barriers to disseminating local findings. Data quality has improved in each cycle of the study but must be protected by continually checking for errors and training field staff to be attentive to detail. Compared with the US census and other convenience samples of Denver MSM, the overall demographic representativeness of NHBS has improved over time. However, there is concern that the underlying population included in the study may be changing. NHBS survey data show evidence of two suspected trends in the local MSM population at risk for HIV: increasing sexual risk-taking and the transition away from bars as a dominant partner-finding location. It is unclear whether the increasing reports of sexual risk-taking reflect a real trend or simply a change in the population sampled, since most NHBS participants are recruited at gay bars and other venues. To ensure that the sample continues to represent the underlying population at risk and accurately identify trends, it is necessary to closely monitor MSM sample characteristics during implementation and incorporate weighted data provided by the Centers for Disease Control and Prevention into analyses.