The development of resistance to pharmacological treatment is common to many human diseases. In chronic epilepsy, many patients develop resistance to anticonvulsant drug treatment during the course of their disease, with the underlying mechanisms remaining unclear. We have studied cellular mechanisms underlying drug resistance in resected hippocampal tissue from patients with temporal lobe epilepsy by comparing two groups of patients, the first displaying a clinical response to the anticonvulsant carbamazepine and a second group with therapy-resistant seizures. Using patch-clamp recordings, we show that the mechanism of action of carbamazepine, use-dependent block of voltage-dependent Na(+) channels, is completely lost in carbamazepine-resistant patients. Likewise, seizure activity elicited in human hippocampal slices is insensitive to carbamazepine. In marked contrast, carbamazepine-induced use-dependent block of Na(+) channels and blocked seizure activity in vitro in patients clinically responsive to this drug. Consistent with these results in human patients, we also show that use-dependent block of Na(+) channels by carbamazepine is absent in chronic experimental epilepsy. Taken together, these data suggest that a loss of Na(+) channel drug sensitivity may constitute a novel mechanism underlying the development of drug-resistant epilepsy.