Previous studies have shown that high pleural fluid (Pf) hyaluronan (HYA) concentrations may be due not only to malignant mesothelioma but also to inflammatory diseases. The objective of this study was to evaluate Pf-HYA in various nonmalignant inflammatory pleural disorders. A radiometric assay was used to determine HYA in Pf and serum (S) of 126 patients, 12 of whom had rheumatoid arthritis (RA), 22 tuberculosis, 22 pneumonia, 41 lung cancer, 10 malignant mesothelioma and 19 congestive heart failure. Pf-HYA values were correlated with values for Pf-tumour necrosis factor (TNF)-alpha and Pf-interleukin (IL)-1beta, as determined by radioimmunoassay. The highest median Pf-HYA (125.6 mg x L(-1), range 0.04-386.5 mg x L(-1)) occurred in patients with malignant mesothelioma. Among patients with nonmalignant inflammatory diseases, significantly higher median Pf-HYA were observed in those with rheumatoid arthritis (64.2 mg x L(-1), range 25.8-106.9 mg x L(-1)) than in those with tuberculosis (25.5 mg x L(-1), range 14.9-57.1 mg x L(-1), p<0.0005) or pneumonia (20.9 mg x L(-1), range 9.5-129.4 mg x L(-1), p<0.005). There was no correlation between Pf-HYA and S-HYA. Pf-HYA correlated positively with Pf-TNF-alpha (r=0.62) and Pf-IL-1beta (r=0.52). High pleural fluid hyaluronan occurs not only in malignant mesothelioma, but also in certain nonmalignant inflammatory diseases, especially rheumatoid arthritis. One explanation for the increase in pleural fluid hyaluronan may be local production of proinflammatory cytokines, such as tumour necrosis factor-alpha and interleukin-1beta.