In recent years, Japanese oak wilt (JOW) has become prevalent in Japan and has spread into almost all prefectures. To understand the effects of a JOW disturbance on nitrate nitrogen (NO3-) concentrations in a forest stream, we analyzed the changes in NO(3)(-)concentrations over a period of 7 years in a mixed broadleaved-conifer secondary forest in Japan. The JOW disturbance arose in the fifth year and lasted until the seventh year of the study period, resulting in a 4.7%, 3.0%, and 2.5% loss in the total basal area of the forest in the fifth, sixth, and seventh years, respectively, eventually resulting in an estimated canopy cover loss of 9.7%. The mean NO3- concentration after the appearance of JOW (3.99 mu mol L-1) was significantly higher than before the start of the disturbance (1.99 mu mol L-1). This study demonstrated that even the modest canopy cover loss by JOW-induced dieback of Quercus serrata trees could cause an increase in stream water NO3- concentration.