We examined the effect of forest thinning on overland flow generation on hillslopes covered by a Japanese cypress plantation in central Japan. We established hillslope plots in two small catchments for monitoring overland flow generation. Plot 1 was a treatment plot in which 58.3% of stems were removed, whereas plot 2 remained untreated as a control. Overland flow generation was examined using (1) time-trend analysis that compared overland flow to precipitation 2 years pre- and 2 years post-thinning in plot 1 and (2) a paired-plot approach that compared the treated hillslope plot to the control hillslope plot. The latter approach was more reliable for identifying treatment effects than time-trend analysis because it minimizes effects of climate variables such as precipitation. Unlike our initial hypothesis for decreasing overland flow after thinning, paired-plot analysis indicated that after thinning, mean overland flow increased 7.4% (corresponding to 2.1 mm) in response to individual storms and 4.8% (corresponding to 7.9 mm) based on monthly data. Although the increased overland flow is likely associated with reduced canopy interception, the reduction of canopy interception after the thinning was estimated to be greater than the increases in overland flow. Therefore, sonic of the increased throughfall was infiltrated into the soil matrix. Such changes in hillslope-scale runoff in response to thinning alter the distribution of water in the surface and subsurface layers of forested headwater basins. Copyright (C) 2010 John Wiley & Sons, Ltd.