A double-blind placebo-controlled trial of 7 days administration of lamotrigine as add-on therapy was performed in 10 patients with frequent therapy-resistant, chiefly partial, seizures. Dosage was adjusted on the basis of estimated half-life. Six patients showed a 50% seizure reduction on lamotrigine and two an increase. Side effects (ataxia, dizziness and apathy) occurred in 3 patients, but only at blood levels above 3 micrograms/ml, and were rapidly relieved when the dose was reduced in two. EEG spike counts were significantly reduced on lamotrigine. There was no evidence of interactions with co-medication.