To examine negative mood as a proximal antecedent and reinforcing condition of binge eating in binge eating disorder (BED) and bulimia nervosa (BN). Using an ecological momentary assessment design, 20 women with BED, 20 women with BN, and 20 nonclinical control women were recruited from the community, provided with a portable minicomputer, and asked to rate their mood and list their thoughts at randomly-generated beep sounds and before, during, and after episodes of eating. In both eating disorder groups mood before binge eating was more negative than before regular eating and at random assessment. Binge eating was followed by a deterioration of mood. The BED group revealed less antecedent negative mood than the BN group and less concomitant negative cognitions about food/eating and stress. Affect regulation difficulties likely lead to binge eating in both disorders, but binge eating may not be effective for regulating overall mood. (c) 2007 by Wiley Periodicals, Inc.