OBJECTIVE: The Diabetes Control and Complications Trial (DCCT) demonstrated the beneficial effects of intensive versus conventional therapy on the development and progression of microvascular complications of type 1 diabetes. These beneficial effects were almost completely explained by the difference between groups in the levels of HbA1c, which in turn were associated with the risk of these complications. We assessed the association of glucose variability within and between quarterly 7-point glucose profiles with the development and progression of retinopathy, nephropathy, and cardiovascular autonomic neuropathy during the DCCT. RESEARCH DESIGN AND METHODS: Measures of variability included the within-day and updated mean (over time) of the SD, mean amplitude of glycemic excursions (MAGE), and M-value, and the longitudinal within-day, between-day, and total variances. Imputation methods filled in the 16.3% of expected glucose values that were missing. RESULTS: Cox proportional hazards models assessed the association of each measure of glycemic variation, as a time-dependent covariate, with the risk of retinopathy and nephropathy, and a longitudinal logistic regression model did likewise for cardiovascular autonomic neuropathy. Adjusted for mean blood glucose, no measure of within-day variability was associated with any outcome. Only the longitudinal mean M-value (over time) was significantly associated with microalbuminuria when adjusted for the longitudinal mean blood glucose and corrected for multiple tests using the Holm procedure. CONCLUSIONS: Overall, within-day glycemic variability, as determined from quarterly glucose profiles, does not play an apparent role in the development of microvascular complications beyond the influence of the mean glucose.