The Diabetes Control and Complications Trial (DCCT) recruited its first patients in 1983. In 1993, the investigators reported that intensive glycemic treatment of patients with type 1 diabetes mellitus was superior to conventional therapy in preventing the development of microvascular and neurological complications and thus provided definitive proof of the relationship between hyperglycemia and the subsequent risk of diabetic retinopathy, nephropathy and neuropathy. The value of this study, however, did not end there. After the original trial, most participants of the DCCT continued to be followed up in the Epidemiology of Diabetes Interventions and Complications (EDIC) study, which demonstrated the long-term benefits of close glycemic control and provided observational data of a large epidemiological cohort of patients with type 1 diabetes mellitus. Stored samples from the DCCT have also provided an invaluable resource for the identification of new markers of the disease. Recently, the complete dataset of the DCCT and of the initial years of the EDIC have been made publicly available, which allowed independent investigators to help answer their own questions about diabetes. In conclusion, the DCCT continues to provide new insights into type 1 diabetes mellitus, which are of benefit to patients over a quarter of a century after the trial was started.