Perceived control over diabetes may serve to buffer the relationship between adolescents' experience of daily negative affect and daily problems with diabetes. In a daily diary study including 209 adolescents (ages 10.5-15.5) with type 1 diabetes, we examined how daily affect related to daily fluctuations in experience of diabetes problems, and whether perceptions of control moderated these daily associations. Using hierarchical linear modelling, we found that day-to-day experiences of negative affect were associated with more frequent daily diabetes problems. Perceptions of treatment control moderated associations between negative affect and number of problems; negative affect was more strongly associated with number of problems among teens perceiving lower versus higher treatment control over their illness. The same pattern of association was not apparent for personal control. Results suggest that perceived treatment control may help to buffer detrimental associations between negative affect and adolescents' ability to successfully manage their diabetes.