This study examined grade and sex patterns in real-time measures of daily mood using Ecological Momentary Assessments via palmtop computers among 8th (N = 296) and 10th graders (N = 266) for 1 year using a three-wave longitudinal design. Participants responded to five to seven random prompts/day for 7 consecutive days; when prompted, participants rated eight mood adjectives assessing positive affect (PA) and negative affect (NA). Global depressed mood was also assessed via self-report questionnaire at every wave. Mixed-effects regression analyses indicated that PA modestly but significantly declined across Grade 8 to 11, but NA was relatively stable over time. Sex by time interactions showed that boys experienced greater declines in daily mood over time than did girls. In contrast, global depressed mood did not change over time, and girls reported higher depressed mood than boys. Findings suggest that normative mood declines in adolescence may be driven by deteriorations in PA, rather than increases in NA.