The 1H Hahn spin-echo NMR profiles of rheumatoid synovial fluids have been investigated and compared with those of matched serum samples. In addition to markedly elevated lactate and diminished glucose concentrations, inflammatory synovial fluids contained (i) substantially lower levels of NMR-detectable chylomicron-and very-low-density-lipoprotein-associated triacylglycerols which appear to have a shortened mean chain-length, and (ii) high concentrations of ketone bodies (predominantly 3-D-hydroxybutyrate), relative to those of corresponding paired serum samples. These observations confirm the abnormal metabolic status of the inflamed rheumatoid joint and provide evidence for an increased utilisation of lipids for fuel therein.