< jats: p> The Diabetes Control and Complications Trial (DCCT) is a multicenter, randomized, clinical study designed to determine whether an intensive treatment regimen directed at maintaining blood glucose concentrations as close to normal as possible will affect the appearance or progression of early vascular complications in patients with insulin-dependent diabetes mellitus (IDDM). We present the baseline characteristics and 1-yr results of the initial cohort of 278 subjects randomized in phase II of the trial, a phase designed to answer several feasibility questions before initiating a full-scale trial.< jats: p> During phase II, recruitment was completed on schedule. The 191 adults and 87 adolescents were randomized either to standard treatment (90 adults and 42 adolescents), designed to approximate conventional diabetes treatment, or to experimental treatment (101 adults and 45 adolescents), designed to achieve near-normal blood glucose and HbA1c concentrations. With few exceptions, baseline demographic, ophthalmologic, renal, and other medical characteristics were evenly distributed by randomization between the two treatment groups in both age strata. Glycemic control at baseline, as assessed by HbA1c concentrations and by blood glucose profiles, was comparable between the treatment groups in both age strata.< jats: p> The treatment strategies employed produced statistically significant and clinically meaningful differences in HbA1c concentrations and blood glucose profiles between the experimental-and standard-group subjects for both adults and adolescents. These differences were maintained throughout the feasibility phase. Except for an increased incidence of hypoglycemia in the experimental group, the two treatment regimens maintained or improved the clinical well-being of subjects in both groups. Adherence and completeness of follow-up were excellent (> 95%), and the methods employed to measure biochemical and pathologic characteristics of IDDM proved to be reliable, reproducible, and precise.< jats: p> The feasibility phase of the DCCT demonstrated that a complex multicenter, randomized study of the relationship between diabetes control and complications can be performed. The full-scale, long-term trial therefore has been initiated.