To assess the effect of itraconazole, a potent inhibitor of cytochrome P450 (CYP) 3A4, on the single oral dose pharmacokinetics and pharmacodynamics of alprazolam, the study was conducted in a double-blind randomized crossover manner with two phases of treatment with itraconazole-placebo or placebo-itraconazole. Ten healthy male subjects receiving itraconazole 200 mg/day or matched placebo orally for 6 days took an oral 0.8 mg dose of alprazolam on day 4 of each treatment phase. Plasma concentration of alprazolam was measured up to 48 h after alprazolam dosing, together with the assessment of psychomotor function by the Digit Symbol Substitution Test, Visual Analog Scale and Udvalg for kliniske undersøgelser side effect rating scale. Itraconazole significantly (P < 0.01) increased the area under the concentration-time curves from 0 h to infinity (252 ± 47 versus 671 ± 205 ng h/ml), decreased the apparent oral clearance (0.89 ± 0.21 versus 0.35 ± 0.10 ml/min per kg) and prolonged the elimination half-life (15.7 ± 4.1 versus 40.3 ± 13.5 h) of alprazolam. The test performed during itraconazole treatment showed significantly depressed psychomotor function. It is suggested that itraconazole, a potent CYP3A4 inhibitor, increases plasma concentration of alprazolam via its inhibitory effects on alprazolam metabolism. Thus, this study supports previous studies suggesting that CYP3A4 is the major enzyme catalyzing the metabolism of alprazolam. Enhanced side effects of alprazolam by itraconazole coadministration were probably reflected by these pharmacokinetic changes.