Context:Impaired vitamin D metabolism may contribute to the development and progression of diabetic kidney disease.Objective:The aim of the study was to test associations of circulating vitamin D metabolites with risks of incident microalbuminuria, impaired glomerular filtration rate (GFR), and hypertension in type 1 diabetes.Design:We performed a cohort study of 1193 participants in the Diabetes Control and Complications Trial (DCCT), a randomized clinical trial of intensive diabetes therapy, and its observational follow-up, the Epidemiology of Diabetes Interventions and Complications (EDIC) Study. We measured plasma concentrations of 25-hydroxyvitamin D [25(OH)D], 1,25-dihydroxyvitamin D, and 24,25-dihydroxyvitamin D by mass spectrometry at the end of the DCCT and tested associations with incident microalbuminuria, impaired GFR, and hypertension over up to 16 yr of EDIC follow-up.Results:At the time metabolites were measured, mean age was 32.4 yr; mean duration of diabetes, 7.5 yr; mean iothalamate GFR, 132.9 ml/min/1.73 m(2); and geometric mean albumin excretion rate, 11.8 mg/24 h. Over follow-up, 166 cases of microalbuminuria, 54 cases of impaired GFR, and 541 cases of hypertension were observed. Compared with 25(OH)D of at least 30 ng/ml, 25(OH)D below 20 ng/ml was associated with a 65% higher risk of microalbuminuria (95% confidence interval, 7 to 154%) in adjusted analyses. Low concentrations of 24,25-dihydroxyvitamin D, but not 1,25-dihydroxyvitamin D, were also associated with increased risk of microalbuminuria. No circulating vitamin D metabolite was associated with risk of impaired GFR or hypertension.Conclusions:Low plasma concentrations of 25(OH)D and 24,25-dihydroxyvitamin D are associated with increased risk of microalbuminuria in type 1 diabetes. In contrast, we did not find evidence linking impaired vitamin D metabolism to early GFR loss or the development of hypertension.