Clomiphene citrate (CC) has been the first line of treatment for induction of ovulation in patients with anovulatory infertility since 1962. It is administered as a mixture of two geometric isomers, Enclomiphene (En) and Zu-clomiphene (Zu) citrate in the ratio 62%:38%. The En isomer is perceived to be more active. A prediction model of response to CC was developed utilizing physical and biochemical characteristics of patients but this model has not been vahdated in a set of patients independent of those used to develop the model. It examines an existent nomogram for the identification of patient resistant to CC therapy and indicates that the nomogram is not sufficiently sensitive and specific to be useful in clinical practice. It shows that obesity could change the pharmacokinetics of Zu but not En. It provides early indications for a hypothesis that therapeutic outcome may be linked to interindividual variation in the exposure to the metabolite formed by CYP2D6. This may explain the lack of any direct relationship between the concentrations of the parent compound and response.