Pranlukast (SB 205312; ONO-1078), a potent, orally active selective cysteinyl-leukotriene receptor antagonist (LTRA), was developed in Japan for the treatment of asthma. This article reports results of the initial U.S. clinical evaluation of pranlukast. The primary objective of this multicenter study was to evaluate the safety and tolerability of pranlukast administered at doses of 337.5 mg b.i.d. and 450 mg b.i.d. in 65 patients with mild to moderate asthma. Pranlukast, a novel LTRA, is safe and well tolerated at doses of 337.5 mg b.i.d. and 450 mg b.i.d. Pranlukast has demonstrated clinical activity in patients with asthma.