The main purpose of this study was to investigate the relationships between affect and both health and lifestyle factors. Japanese undergraduates (223 men and 302 women) completed the Positive and Negative Affect (PA and NA) Schedule scales, the General Health Questionnaire, and the Health Behavior Questionnaire in order to measure affect, mental and physical health, and lifestyle, respectively. These three measures were administered twice (T1 and T2), five or six weeks apart. Hierarchical regression analyses of the data showed that better T2 health status was significantly associated with lower T1 NA in men and both higher T1 PA and lower T1 NA in women. Meanwhile, lifestyle factors at T2 had few significant associations with PA and NA at T1. A number of significant interactions of PA × NA suggested that higher T1 PA was only associated with better health and lifestyle factors at T2 when T1 NA was low. The findings indicate that both PA and NA, including PA × NA interactions and gender differences, have important health implications.