Hooking up (sex outside committed, romantic relationships) is feared to result from or lead to compromised psychological well-being among undergraduates, yet longitudinal evidence is scarce and inconclusive, and different hookup definitions complicate cross-study comparisons. This study examined short-term longitudinal associations with four well-being indicators (depression, anxiety, life satisfaction, and self-esteem) across several definitions of hookups based on relationship length (one time, longer casual, and any) and physical intimacy level (kissing, genital touching, oral sex, and intercourse). A university-wide sample of 666 Northeastern U.S. freshmen and juniors (63% female, 68% White) completed online surveys at the beginning and end of one academic semester. Linear and logistic regressions explored whether hookups over the semester were linked to later well-being, and whether initial well-being was linked to later hookups. Across all 96 regressions, statistically significant associations between well-being and hookups were infrequent (23%), predominantly confined to anxiety and life satisfaction, equally likely in the direction of higher (13%) as lower (10%) well-being, and affected by both casual relationship length and intimacy level. When gender differences emerged (11%), hookups were associated with higher well-being for women and lower well-being for men. This complex set of results points to the importance of researchers' choices in hookup definitions.