The Diabetes Control and Complications Trial (DCCT) is a prospective randomized multicenter clinical trial to compare the effects of intensive insulin therapy aimed at near normalization of blood glucose levels with the effects of conventional therapy on the early microvascular complications of insulin-dependent diabetes mellitus. With the successful completion of its planning and feasibility phases, the DCCT is currently conducting the full-scale long-term trial. The total study cohort (n = 1441) has been recruited and randomized to either intensive or standard therapy. The range of follow-up for all subjects will be 5-10 yr, with a median of 7 yr at the trial's end in 1993. Adherence to the study protocol and prescribed regimens remains excellent. Glycemic separation between the randomized treatments, demonstrated during the 1-yr feasibility phase, remains over the first 3 yr of the study. This update reviews the rationale and design of the DCCT, changes in the protocol since its inception, study progress to date, including baseline characteristics of the entire study population, and the results of analyses of baseline data.