Hook ups are casual sexual encounters (ranging from kissing to intercourse) between two people with no clear mutual expectation of further interactions or a committed relationship. This study utilized a short-term prospective design to examine predictors of hooking up in a sample of young adults (N = 394). Hooking up over the past year, positive reactions to prior hook ups, alcohol use, and loneliness were associated with hooking up over a 4-month period. Alcohol use was a stronger predictor for women than men. Thoughtfulness about relationship transitions and religiosity were significant predictors of hooking up in univariate analyses, but were not significant in multivariate analyses. Young adults who reported more depressive symptoms and feelings of loneliness at Time 1 and subsequently engaged in penetrative hook ups reported fewer depressive symptoms and lower feelings of loneliness at Time 2 as compared to young adults who did not hook up. However, young adults who reported fewer depressive symptoms and were less lonely at Time 1 and engaged in penetrative hook ups over the 4 month period reported more depressive symptoms and greater feelings of loneliness at Time 2 as compared to young adults who did not hook up. Implications for relationship education programs are offered.