This study focuses on the sexual phenomenon of "hooking up." A hook-up is defined as a single sexual encounter that may or may not include sexual intercourse with someone who is a stranger, brief acquaintance, or friend. The aim of this study was to document the prevalence of hook-ups in a sample of 1,011 urban middle and high school students and to examine the relationship between hooking up and a variety of problem behaviors, including, alcohol, cigarette, and/or illicit drug use, truancy, and school suspensions. The results revealed that 28% of the sample had engaged in at least one hook-up experience, and this percentage increased with age. Hook-ups were correlated moderately with all problem behaviors examined. (Contains 4 tables.)