13C NMR spectra of human surgical muscle samples have been obtained at 50 and 118 MHz. Numerous sharp peaks in the 13C spectrum have been assigned to carbon atoms of soluble metabolites and fatty acyl chains of neutral fats and membrane-bound phospholipids. Comparisons have been made of 13C NMR spectra of normal and diseased muscles after removal of neutral fat by extraction with isopentane. Creatine, lactic acid, and phospholipids are not removed from muscles by isopentane. The most striking difference between 13C NMR spectra of isopentane-extracted normal and diseased muscle samples was the size of the residual 30.5 ppm methylene carbon resonance, that is, small in normal muscles and in nonspecific muscle diseases, but large in myogenic and neurogenic muscle diseases. In addition, differences were also found between normal and diseased muscles in their creatine content and their ability to produce lactic acid. By deoxycholate treatment of isopentane-extracted diseased muscle it is estimated that about one-fifth of the total phospholipids are highly mobile. T1 and NOE measurements indicated that the differences in peak height for the 30.5 ppm resonance between normal and diseased muscle are due only to difference in the amount of highly mobile fatty acyl chains in the muscle and not due to differences in relaxation parameters. 13C NMR of isopentane-extracted muscle appears to permit differentiation of normal from diseased muscle and, within diseased muscle, grading of the severity of the disease.