The magnetic resonance imaging–estimated proton density fat fraction (MRI-PDFF) is a novel imaging-based biomarker that allows fat mapping of the entire liver, whereas the magnetic resonance spectroscopy–measured proton density fat fraction (MRS-PDFF) provides a biochemical measure of liver fat in small regions of interest. Cross-sectional studies have shown that MRI-PDFF correlates with MRS-PDFF. The aim of this study was to show the utility of MRI-PDFF in assessing quantitative changes in liver fat through a three-way comparison of MRI-PDFF and MRS-PDFF with the liver histology–determined steatosis grade at two time points in patients with nonalcoholic fatty liver disease (NAFLD). Fifty patients with biopsy-proven NAFLD who participated in a randomized trial underwent a paired evaluation with liver biopsy, MRI-PDFF, and MRS-PDFF at the baseline and 24 weeks. The mean age and body mass index were 47.8 ± 11.7 years and 30.7 ± 6.5 kg/m2, respectively. MRI-PDFF showed a robust correlation with MRS-PDFF both at week 0 and at week 24 (r = 0.98, P < 0.0001 for both). Cross-sectionally, MRI-PDFF and MRS-PDFF increased with increases in the histology-determined steatosis grade both at week 0 and at week 24 (P < 0.05 for all). Longitudinally, patients who had a decrease (≥1%) or increase (≥1%) in MRI-PDFF (confirmed by MRS-PDFF) showed a parallel decrease or increase in their body weight and serum alanine aminotransferase and aspartate aminotransferase levels at week 24 (P < 0.05). This small increase or decrease in liver fat could not be quantified with histology. Conclusion: In this longitudinal study, MRI-PDFF correlated well with MRS-PDFF and was more sensitive than the histology-determined steatosis grade in quantifying increases or decreases in the liver fat content. Therefore, it could be used to quantify changes in liver fat in future clinical trials. (Hepatology 2013; 58:1930–1940)