In April 1990, the El Paso County (Colorado) Health Department (EPCHD) recognized an outbreak of penicillinase-producing Neisseria gonorrhoeae (PPNG) and other sexually transmitted diseases (STDs) occurring in Colorado Springs (1990 census population: 397,014), Colorado. An investigation by the EPCHD and the Colorado Department of Health eventually identified 56 cases of PPNG from December 1989 through March 1991 (Figure 1). The outbreak revealed a previously unidentified core group of persons with STDs in Colorado Springs. This report summarizes traditional and innovative measures used to investigate and manage the outbreak, and describes this core group and its role in STD transmission in Colorado Springs.