Several theories posit that cannabis and other substances are used to reduce negative affect. This daily report study considered whether variations in positive and negative affect, reported each morning, contributed to the likelihood of cannabis use later that day. We also explored whether levels of positive and negative affect reported immediately after cannabis use improved, relative to that day's morning levels. The sample included 183 men and 183 women representing heterosexual, cannabis-using couples from the community. Participants made independent, daily reports of affect and cannabis use episodes for 30 consecutive days. Using multilevel modeling, we modeled men's and women's use of cannabis on a given day as a function of morning levels of positive, hostile, and anxious affect, accounting for partner cannabis use that day, and mean levels of positive and negative affect. Men and women were more likely to use cannabis on a given day when morning positive affect was lower than typical for the person and when partner used cannabis that day. Neither hostile nor anxious affect contributed to later use of cannabis. Immediately after cannabis use, positive affect increased, and hostile and anxious affect decreased relative to that day's morning levels. The improved affect immediately after use suggests a mechanism of positive reinforcement.