When monitoring of the plasma concentrations of anticonvulsant drugs first came into use 25 years ago, it appeared to have a major impact on improving the effectiveness and safety of anticonvulsant therapy. However, as time has passed, prescribers have absorbed many of the lessons to be learned from the monitoring, and now apply this knowledge without necessarily monitoring plasma anticonvulsant concentrations as frequently as in the past. Therefore, the effect of the drug concentration monitoring on the cost effectiveness of anticonvulsant therapy is probably not as significant now as it originally was. In theory, drug concentration monitoring is often unlikely to decrease the cost of contemporary anticonvulsant drug therapy, but it may enhance the efficacy of the therapy. Thus, monitoring may reveal unrecognised under- or overdosage, detect failure of compliance or drug-drug interactions, or indicate when there is little point in persisting with a particular anticonvulsant drug. Despite a good deal of anecdotal testimony, surprisingly little has been published demonstrating the benefits of anticonvulsant therapeutic drug monitoring in epileptic populations. However, one study did show better rates of seizure control rates in patients monitored in the first 6 months of their epileptic disorder; but not if the monitoring began later than this.(ABSTRACT TRUNCATED AT 250 WORDS)