Leukotrienes (LK) play an important role in the inflammatory response found in the asthmatic airway and it is therefore of interest to explore the clinical value of agents which can either block the effect or inhibit the synthesis of leukotrienes. If tachyphylaxis does not develop, repeated LTD4 challenges can be used for dose ranging studies. Tachyphylaxis has been observed when LTD4 challenge intervals are kept below 1 h, but the effect of prolonging the interval is uncertain. The aim of this study was therefore to investigate the interval between LTD4 challenges necessary to avoid development of tachyphylaxis. Ten stable adult asthmatics with moderate asthma were challenged seven times: a screening day to secure responsiveness to LTD4, two challenges on three separate days with an interval of 2, 4 and 6 h, respectively. No significant differences between mean PC20-LTD4 values (P > 0.5) was found. It was concluded that tachyphylaxis to repeated LTD4 challenges of asthmatics does not develop when challenge intervals are prolonged above 2 h.