In VITAL, 25,875 participants have been randomly assigned in a 2x2 factorial design to vitamin D3 (cholecalciferol) 2000 IU daily versus placebo, and to eicosapentaenoic acid 465 mg plus docosahexaenoic acid 375 mg daily versus placebo, and followed for a mean of 5 years to assess effects on cardiovascular disease and cancer events. This ancillary study will identify and recruit a sub-cohort of VITAL participants with hypertension at baseline and ascertain effects of study interventions on glomerular filtration rate in this group. Blood samples will be collected at year 4 simultaneously for measurement of estimated glomerular filtration rate (using serum creatinine and cystatin C) and other relevant biomarkers. This VITAL ancillary study is designed to determine whether vitamin D3 and/or omega-3 fatty acids have causal and clinically relevant effects on the development and progression of kidney disease in hypertensives.