Summertime observations of O3and CO made at the PICO-NARE station during 2001, 2003, and 2004 are used to assess the impact of boreal forest fires on the distribution of O3 mixing ratios in the midlatitude Northern Hemisphere (NH) lower free troposphere (FT). Backward trajectories were used to select measurements impacted by outflow from high-latitude regions. Measurements during these periods were segregated into two subsets: those obtained during periods with and without apparent significant upwind fire emissions. Periods affected by fire emissions were identified based on enhanced CO levels confirmed by global simulations of fire emissions transport. During fire-impacted periods, O3 was shifted toward higher mixing ratios, with medians significantly higher than in periods without detectable upwind fire impacts. This implies a significant impact of boreal wildfires on midlatitude lower FT background O3 during summer. Predicted future increases in boreal wildfires may therefore affect summertime O3 levels over large regions. Copyright 2006 by the American Geophysical Union