Objectives. This study assessed factors related to chlamydial infection among young men in the United States. Methods. Data were from interviews of nationally representative samples of 470 men aged 18 to 19 years (teenagers) and 995 men aged 22 to 26 years (young adults) and from urine specimens tested by means of polymerase chain reaction. Results. Although a majority of the men reported occasional unprotected intercourse, only a minority perceived themselves to be at risk for contracting a sexually transmitted disease (STD). Chlamydial infection was detected in 3.1% of the teenagers and 4.5% of the young adults. A minority of those infected had symptoms or had been tested for STDs; very few had been diagnosed with STDs. Conclusions. Chlamydial infection is common but usually asymptomatic and undiagnosed. Primary and secondary prevention efforts should be increased, particularly among young adult men. (Am J Public Health. 2002;92:11401143)