At present, only one mutation of KCNQ3, a KCNQ potassium channel gene, has been identified as a cause of benign familial neonatal convulsions type 2 (BFNC2). We found a T to C substitution (c.925T-C) on one allele of affected individuals in a Japanese family with BFNC but not on 200 alleles from healthy subjects. c.925T-->C replaced Trp309, a conserved residue within the P-loop of the KCNQ potassium channel family that holds the channel pore open, with an Arg (W309R). We report c.925T-->C as the second mutation of KCNQ3 responsible for BFNC2.