Objectives : Serum and plasma BDNF levels have been shown to be decreased in patients with mood disorder such as major depressive disorder and bipolar disorder. We investigated whether platelet BDNF levels would be lower in patients with acute bipolar manic episode compared with those of normal controls. Methods : BDNF levels were examined in platelet-rich plasma(PRP) and platelet-poor plasma(PPP) in 20 healthy controls and 20 hospitalized patients who were diagnosed as bipolar I disorder, most recent episode manic using a Structured Clinical Interview for DSM-IV. And severity of manic symptoms was measured using Young Mania Rating Scale(YMRS). Platelet BDNF level was calculated by subtracting PPP BDNF from PRP BDNF level, and dividing the result by the total platelet count, and it was expressed as pg/$10^6$ platelet. Results : Platelet BDNF levels were significantly lower in patients with acute bipolar manic episode(4.55${pm}$3.36pg/$10^6$ platelet) than in normal controls(6.84${pm}$2.32pg/$10^6$ platelet)(p=0.008). However we failed to reveal the significant negative correlation between platelet BDNF levels and YMRS scores in patients with acute bipolar episode. Conclusion : Our finding suggests that there is a decrease in the platelet BDNF of patients with acute bipolar manic episode.