We propose a randomized, placebo-controlled clinical trial of vitamin D3 (cholecalciferol) and omega-3 fatty acids (eicosapentaenoic acid plus docosahexaenoic acid) to prevent the development and progression of diabetic kidney disease (DKD). Persons with diabetes are at high risk of developing kidney disease, and DKD is both the leading cause of end stage renal disease in the developed world and a potent amplifier of cardiovascular disease risk. Vitamin D and omega-3 fatty acids are promising interventions for DKD prevention and treatment, based on results of animal-experimental models and early human studies. Because these interventions are relatively safe, inexpensive, and widely available, they may offer opportunity to substantially reduce the burden of DKD in large populations. We aim to test whether vitamin D3 and/or omega-3 fatty acids prevent progression of albuminuria and loss of glomerular filtration rate, two complementary manifestations of DKD, over 4 years of treatment. To effectively and efficiently test study hypotheses, we propose an ancillary study to the Vitamin D and Omega-3 Trial (VITAL), an NIH-funded randomized clinical trial. In VITAL, 20,000 participants (men ages e 60 years, women ages e 65 years) will be randomly assigned in a 2x2 factorial design to vitamin D3 (cholecalciferol) 1600 IU daily versus placebo, and to eicosapentaenoic acid 500 mg plus docosahexaenoic acid 500 mg daily versus placebo, and followed for a mean of 5 years to assess effects on cardiovascular disease and cancer events. Leveraging the established infrastructure of VITAL, we propose to identify and recruit a sub-cohort of 1,500 VITAL participants with diabetes at baseline and to ascertain effects of study interventions on albuminuria and glomerular filtration rate in this group. Following the simple and cost-efficient design of the parent VITAL trial, biospecimens will be collected locally and delivered to the VITAL central laboratory by mail. First morning voids will be collected at baseline and year 4 for measurement of urine albumin-creatinine ratio. Blood samples will be collected simultaneously for measurement of glomerular filtration rate (using serum creatinine and cystatin C), 25- hydroxyvitamin D, eicosapentaenoic acid plus docosahexaenoic acid, C-reactive protein, and hemoglobin A1c. The proposed studies are designed to determine whether vitamin D3 and/or omega-3 fatty acids have causal and clinically relevant effects on the development and progression of DKD.