Abstract: A method of determining or imaging fats in a non .sup.13 C enriched biological sample, particularly human tissue comprising subjecting the sample to a magnetic field, irradiating the sample with radiofrequency electromagnetic radiation, obtaining from the sample an electrical signal indicative of the radiofrequency absorption of the sample in the magnetic field at a frequency corresponding to the .sup.13 C NMR resonance frequency for --(CH.sub.2).sub.n --, filtering the signal to exclude components due to absorption by the sample attributable to carbon atoms other than those present as --(CH.sub.2).sub.n -- and determining from the intensity of the absorption the fat content of the sample, or applying to the signal an imaging technique to determine the fat distribution in the sample. .sup.1 H decoupling is preferably employed to increase signal-to-noise ratio.The method is useful in the diagnosis of certain fat-related diseases, such as muscular dystrophy.