During 2001-2006, new human immunodeficiency virus (HIV) diagnoses among black men aged 13-24 years who have sex with men (MSM) in 33 states increased by 93%. The Wisconsin Division of Public Health (WDPH) recently reported to CDC a 144% increase during 2000-2008 in HIV diagnoses among black MSM aged 15-29 years in Milwaukee County. In October 2009, the City of Milwaukee Health Department (MHD), WDPH, and CDC investigated whether the increase in HIV infections among young black MSM in Milwaukee represented increased HIV transmission or simply better identification of prevalent infections. This report describes the results of that investigation, which indicated that a new "social networks" HIV testing strategy and the recent expansion of better targeted HIV testing efforts accounted for few diagnoses among young black MSM and occurred after HIV diagnoses increased, respectively. Therefore, although some diagnoses were made because of intensified testing, an increase in HIV transmission likely occurred. Moreover, an increase in syphilis diagnoses among young black MSM in Milwaukee preceded the increase in HIV diagnoses, which suggests that changes in risk behavior or sexual networks might explain the increase. These findings highlight the need for new or improved interventions promoting prevention education, early HIV detection, and entry to care for young HIV-infected and at-risk black MSM in Milwaukee.