1 H NMR images of human or animal tissues reflect the spatial distribution of both water (H 2 O) and methylene (CH 2) proton resonance signals. There are several reasons for a separation of these contributions:(i) the large chemical shift dispersion in high magnetic fields (> or= 1.5 T) which leads to an apparent spatial shift in'composite'images between the superimposed H 2 O and CH 2 images;(ii) the evaluation and interpretation of proton H 2 O and CH 2 relaxation times from NMR images; and (iii) the physiological implications of'water'and'fat'distributions for medical diagnosis. The authors describe a chemical shift selective (CHESS) imaging technique which destroys the unwanted signal component by means of a selective 90 degrees excitation pulse and a subsequent magnetic field gradient ('homogeneity spoiling gradient') prior to imaging of the wanted component. The new method allows the creation of either a pure'water'or'fat'image.