A new method to assess noninvasively the proportionate storage of poly- and mono-unsaturated fatty acids in human adipose tissue is described. The method is based on in vivo natural abundance 13C NMR at 1.9 T using a TMR 32/200. Data were acquired with a concentric coil system placed below subcutaneous adipose tissue of the calf or the lower arm. Continuous single-frequency decoupling was used to optimize the signal in the spectral region of interest and minimize potential heating effects. Only adipose triglycerides contributed significantly to the resonance intensities. Typical linewidths for the in vivo 13C resonances were 10-14 Hz resulting in excellent resolution in the region of the unsaturated carbons. Signal-to-noise ratios for the resonance at 128.4 ppm (due to carbon atoms of polyunsaturated fatty acids) were typically 50-90 in 8 min using 0.25s repetition time. The relative storage of polyunsaturated fatty acids was determined by comparing the peak intensity at 128.4 ppm with the intensities of the peaks due to the carbonyl, the glycerol, or the main methyl carbons. High-resolution in vitro NMR of adipose biopsies and various mixtures of triglycerides studied with the in vivo NMR protocols provided further evidence that the described in vivo NMR methods allowed precise information on the relative storage of linoleic acid, the main constituent of the pool of polyunsaturated fatty acids. Conventional gas chromatography was performed on adipose biopsies of two volunteers and resulted in close agreement with the in vivo NMR methods. The average polyunsaturated fatty acid concentration in adipose tissue in seven healthy volunteers was 19.83 mol% +/- 1.74 (SD) as determined by in vivo NMR, consistent with conventional studies based on gas chromatography of biopsied material. Preliminary data on three cystic fibrosis patients showed a decreased polyunsaturated fatty acid concentration of 13.12 mol% +/- 2.73 (SD). The high accuracy of this method and its noninvasive character should be especially valuable in clinical research and diagnosis as repetitive adipose biopsies can be avoided.