Precision and accuracy of the quantitative magnetic resonance (QMR) system for measuring fat in phantoms and total body fat (TBF) in humans were investigated. Measurements were made using phantoms: oil, beef with water, beef with oil, and humans with oil and water. TBF(QMR) in humans was compared with TBF by a four-compartment model (TBF(4C)). The coefficient of variation (CV) for replicate TBF(QMR) was 0.437%. QMR fat was lower at 23 °C vs. 37 °C. The fat increase in QMR phantom studies was consistent with the oil increase. When oil was added with humans, the increase in TBF(QMR) was >250 g for the initial 250 g of oil. With additional oil increments, the increase in TBF(QMR) was consistent with the amount of oil added. When water was added with humans, the TBF(QMR) increased independent of the amount of water added. TBF(QMR) was significantly less (mean ± s.e.) than TBF(4C) (females: -0.68 ± 0.27 kg, males: -4.66 ± 0.62 kg; P = 0.0001), TBF(BV) (females: -1.90 ± 0.40 kg; males: -5.68 ± 0.75 kg; P = 0.0001), and TBF(D2O) for males, but greater for females (1.19 ± 0.43 kg vs. -3.69 ± 0.81 kg for males; P = 0.0003). TBF(QMR) was lower than TBF(iDXA) with the difference greater in males (P = 0.001) and decreased with age (P = 0.011). The strong linear relationships between TBF(QMR) and TBF(4C), TBF(BV), and TBF(D2O) with slopes consistent with unity suggest that modifications are required to improve the accuracy. Should the latter be accomplished, QMR holds promise as a highly precise, rapid, and safe, noninvasive method for estimating the amount of and changes in TBF in overweight and severely obese persons.