Lamotrigine (LTG) is an antiepileptic drug that is metabolized via glucuronidation. Since the glucuronidizing enzyme is inducible by estrogens, LTG serum concentrations may fall by 50-60% when combined with hormonal contraceptives that contain ethinyl estradiol (EE). Little is known about a possible interaction between estrogens used for hormone replacement therapy (HRT) and LTG, and the few available data are conflicting. Data from serum samples analyzed for LTG were therefore retrieved from a routine therapeutic drug monitoring database. Users of HRT and EE were identified and matched with controls for age and dose. No enzyme-inducing or enzyme-inhibiting comedication was allowed. LTG serum concentration-to-dose ratios (CDRs) were calculated. Case groups and their respective control groups were compared by the Mann-Whitney U test. Seventy-nine HRT users (dose range 1-4 mg/day) and 200 EE users (dose range 20-40 g/day), as well as 158 and 400 matching controls, respectively, could be included. Both EE users and HRT users had significantly lower mean LTG CDRs than their respective matched controls. These results suggest that HRT with estrogens may reduce serum LTG concentrations.