Background: Transient outward current(Ito) has been known to be reduced in the atrium with atrial fibrillation (AF). Kv4.3 is responsible for the major part of Ito current in the human atrium, and the expression levels as well as the biophysical properties of Kv4.3 are modulated by KChIP. Expression level of KChIP has not been yet elucidated in human AF. Methods and Results: A total of 17 patients undergoing open heart surgery were included. There were 8 men and 9 women with a mean age of 57 짹 14 years (range 19 to 77). Twelve patients had sinus rhythm (SR) and five patients had AF. A piece of right atrial appendage tissue of 0.5 to 1.0 g was obtained during the surgical procedure. Reverse transcriptase-polymerase chain reaction and western blot analysis were performed for the evaluation of the expression status of Kv4.3 and KChIP2. AF group showed significantly reduced KChIP2 expression, however Kv4.3 was not significantly reduced. This is the first report on the KChIP2 status in human AF. Conclusions: KChIP2 is downregulated in human chronic AF. Decreased Ito that observed in AF may be related to the decreased expression of KChIP2 rather than Kv4.3.