Individuals with insulin-dependent diabetes mellitus (IDDM) and their healthcare practitioners believe that extreme blood glucose (BG) fluctuations are characterized by changes in subjective mood states and emotional behavior, as well as physical symptoms. This study examined relationships between BG levels and self-reported mood in a group of 34 IDDM adults. The method followed a within-subject, repeated-measures design employed in previous studies of physical symptoms associated with diabetic glucose. Four times each day, participants completed a mood/symptom checklist just prior to a self-measurement of BG until 40 checklists had been completed. Half the items on the checklist described physical symptoms and half described mood states. In addition, half the mood items described negative states and half described positive states. Within-subject correlations and regressions showed that moods were related to BG for the majority of participants and that, like physical symptoms, mood-BG relationships were highly idiosyncratic. Low BG levels tended to be associated with negative mood states, primarily self-reported "nervousness." Positive mood items were almost always associated with high BG. High BG levels also frequently correlated with negative mood states, although the negative mood items that tended to relate to high glucose (anger, sadness) differed from those that tended to relate to low BG. The implications of these findings for self-treatment and glucose perception in the IDDM individual are discussed.