Volatile short-chain fatty acids (SCFA) in synovial fluid from 80 patients were quantified by gas-liquid chromatography (GLC). Characteristic patterns of volatile SCFA could not be associated with septic, nonseptic inflammatory, or noninflammatory groups. Mean concentrations of pentanoic and hexanoic acids were similar in all groups studied. In the septic arthritis group 3 of 4 patients with acetic acid and of 3 of 3 with 3-methyl butanoic acid had culture-proved staphylococcal infections. In patients with synovial fluid findings consistent with septic arthritis, including markedly raised leucocyte count, decreased glucose level, or detectable succinic acid, the detection of acetic and 3-methyl butanoic acids by GLC analysis may increase the suspicion that Staphylococcus aureus is the cause of the septic arthritis.