The influence of carbamazepine (CBZ) dose, CBZ preparation used, comedication (phenobarbital, phenytoin, primidone, valproate), and factors such as age, weight, and sex on the concentration of CBZ and its metabolites carbamazepine-10,11-epoxide (CBZ-epoxide) and 10,11-dihydro-10,11-dihydroxy-carbamazepine (CBZ-diol) in serum was investigated. A non-linear regression analysis using the data of 609 patients shows that other anti-epileptic drugs can influence the metabolism of CBZ in various ways. The mean serum concentration of CBZ is lower when the drug is given in combination with phenytoin (59.4%), primidone (58.2%), phenobarbital (65.7%), and valproate (83.0%) than when CBZ is given alone (100%), whereas the mean concentration of CBZ-epoxide is increased by valproate (144.8%), by primidone (118.5%), and by a combination of the latter (167.4%). The CBZ-diol concentrations are also increased during concomitant treatment with the other antiepileptic drugs. Our results indicate a nonlinear relationship between the CBZ dose and the CBZ concentration, but a linear relationship between the CBZ dose and the CBZ-diol concentration.