Context: Controversy persists over: 1) how best to restore low serum 25-hydroxyvitamin D (25D) levels (vitamin D2 D2 vs vitamin D3 D3); 2) how best to define vitamin D status (total protein-bound + free vs free 25D); and 3) how best to assess the bioactivity of free 25D. Objective: To assess: 1) the effects of D2 vs D3 on serum total and free 25D; and 2) whether change in intact PTH (iPTH) is more strongly associated with change in total vs free 25D. Design: Participants previously enrolled in a D2 vs D3 trial were matched for age, body mass index, and race/ethnicity. Participants received 50 000 IU of D2 or D3 twice weekly for 5 weeks, followed by a 5-week equilibration period. Biochemical assessment was performed at baseline and at 10 weeks. Setting and Participants: Thirty-eight adults (19 D2 and 19 D3) ≥18 years of age with baseline 25D levels <30 ng/mL were recruited from an academic ambulatory osteoporosis clinic. Outcome Measures: Serum measures were total 25D, free 25D (directly measured), 1,25-dihydroxyvitamin D, calcium, and iPTH. Urine measure was fasting calcium:creatinine ratio. Results: Baseline total (22.2 ± 3.3 vs 23.3 ± 7.2 ng/mL; P = .5) and free (5.4 ± 0.8 vs 5.3 ± 1.7 pg/mL; P = .8) 25D levels were similar between D2 and D3 groups. Increases in total (+27.6 vs +12.2 ng/mL; P = .001) and free (+3.6 vs +6.2 pg/mL; P = .02) 25D levels were greater with D3 vs D2. Percentage change in iPTH was significantly associated with change in free (but not total) 25D, without and with adjustment for supplementation regimen, change in 1,25-dihydroxyvitamin D, and change in calcium. Conclusions: D3 increased total and free 25D levels to a greater extent than D2. Free 25D may be superior to total 25D as a marker of vitamin D bioactivity. 500,000 IU of vitamin D2 or D3 was given over 10 weeks. Total and free 25-hydroxyvitamin D (25D) increased more robustly with D3. Change in iPTH was associated with change in free, but not total, 25D.