CYP3A is one of the most important cytochrome P450 isoforms responsible for drug metabolism by humans because it is the major such enzyme in critical tissues such as the gastrointestinal tract and liver, and it is involved in the oxidative biotransformation of numerous clinically useful therapeutic agents. Many factors regulate CYP3A expression but these are being increasingly defined so that the disposition characteristics of a drug whose metabolism is importantly mediated by this isoform can be reasonably well predicted a priori. For example, metabolic clearance is distributed within a population in a unimodal fashion but marked (5- to 20-fold) interindividual variability is present as a consequence of both genetic and nongenetic factors. In addition, first-pass metabolism occurs following oral drug administration and this may be extensive so that bioavailability is low. CYP3A activity can also be readily modulated by inducers like rifampicin and several anticonvulsant agents, and many potent inhibitors exist such as azole antifungal agents and macrolide antibiotics. Accordingly, the potential for drug interactions with these drugs as well as other CYP3A substrates, when given concomitantly, is high. Metabolism involving CYP3A is also likely to be affected by liver disease as well as aging, and modest differences may be present between men and women but these are often clinically unimportant. Because of such predictability, knowledge of the role and importance of CYP3A in the metabolism of a putative drug candidate is becoming increasingly desirable at an early stage in the development process. In vitro studies using human liver preparations, including microsomes, cultured hepatocytes and heterologous expressed enzymes, can provide important insights in this regard. This is particularly the case for identifying potential drug interactions whose clinical significance can be subsequently assessed. Data with respect to terfenadine and cyclosporine obtained several years after their approval and marketing, indicate that, if available and applied during their development, the paradigm of using in vitro studies to rationally direct and prioritize clinical studies would have prospectively prevented the serious adverse effects and inefficacy that were only recognized during their empiric clinical use. Such examples, along with those associated with the genetic polymorphism of CYP2D6, provide strong justification for establishing the role and importance of individual CYP isoforms in a candidate drug's metabolism at an early stage.