BACKGROUND Leukotriene B4 (LTB4) serum and plasma concentrations were reported to be higher in some asthmatic patients than in normal subjects; however, reported LTB4 concentrations in normal subjects vary widely. One study suggested that blood clotting causes the increased LTB4 concentration. OBJECTIVE To determine whether LTB4 concentration is increased in asthmatic patients, and whether it is affected by clotting. METHODS We studied seven normal subjects and nine clinically stable asthmatic patients. Venous blood was drawn into test tubes without additives; containing heparin; or containing heparin and cyclo- and lipoxygenase inhibitors. Cells were separated after 30 minutes. Leukotriene B4 was measured by radioimmunoassay following its extraction from serum or plasma. In three subjects, plasma was separated also at times 0 through 30 minutes. RESULTS Serum and plasma concentrations of LTB4 in normal volunteers and asthmatic patients were similar, but the variance of LTB4 concentrations among the asthmatic patients was significantly higher than in the normal subjects. Leukotriene B4 concentrations, measured in plasma only, were significantly reduced in both asthmatic and nonasthmatic subjects in the presence of inhibitors. There was no significant difference in LTB4 concentrations between time 0 and 30 minutes, but there was considerable variability. CONCLUSIONS We conclude that clotting is unlikely to affect serum LTB4 concentrations. Leukotriene B4 serum and plasma concentrations are not consistently increased in asthmatic patients; however, LTB4 is synthesized during and possibly after blood has been drawn. Proper handling of the specimens and probably the addition of cyclo-oxygenase and lipoxygenase inhibitors is of the utmost importance for accurate LTB4 determination.