Experts from epileptology and psychiatry reviewed the current significance of therapeutic drug monitoring (TDM) of antiepileptic drugs and psychiatric drugs in a workshop at Bethel Epilepsy Centre in December 2005. TDM has been essential in epileptology for about 30 years, and it is also increasingly important in psychiatry, in which consensus recommendations were published recently. With regard to cost-cutting in the health system, there are discussions about the financial effect of TDM and outsourcing it to bigger laboratories. In psychiatry it has however been shown that sensibly used TDM may lead to reduced costs. Many issues in TDM require the knowledge and experience of specialised laboratories. The use of TDM data for scientific purposes was discussed at the workshop as well.