Several studies have investigated the association between the ABCB1 polymorphism and drug-resistant epilepsy. However, the effect of ABCC2 polymorphisms on anti-epileptic drug (AED) responsiveness remains unknown. The ABCC2 polymorphisms have been genotyped in 279 Japanese epileptic patients treated with AEDs. The association between the AED responsiveness and the polymorphisms was estimated by a haplotype-based analysis. No genotype or haplotype was associated with drug-resistant epilepsy. On the other hand, the delGCGC haplotype at G-1774delG, C-24T, G1249A and C3972T was over represented among the epileptic patients with a complication of mental retardation in comparison with those without (32.4% vs 22.0%; P=0.009); and the G-1774delG allele was also associated with mental retardation (P=0.03). No association between the ABCC2 genotypes or haplotypes, and the responsiveness of AEDs was observed, although this finding was inconclusive because of the small sample size.