Triacylglycerols in human neutrophils exposed to proinflammatory stimuli generate a high-resolution proton magnetic resonance (1H MR) spectrum. Lipid cross-peak F volumes in neutrophils from patients with inflammatory conditions were measured. Values in patients hospitalized with localized infections (14.4 ± 9.0; mean ± SD) or bacteremia (19.3 ± 9.7) were significantly higher than in patients with noninflammatory conditions (6.2 ± 5.3) and healthy controls (2.0 ± 3.0; P <.001). The positive predictive value of F volumes >10 was 93% for all infection; the negative predictive value of volumes ⩽10 was 68% for all infection and 92% for bacteremia. Plasma lipopolysaccharide (LPS) concentrations were highest in bacteremic patients but did not correlate with levels oftumor necrosis factor-α (TNFα) or interleukin-6. In vitro, LPS increased F volumes of control neutrophils from 2.0 ± 3.0 to 37.2 ± 6.7 (P <.001); TNFα had no effect. F volumes in 1H MR spectra may be useful clinically to discriminate between serious bacterial infection and other inflammatory conditions. TNFα is not the stimulus for generation of lipid spectra in vivo.