Ozone is the most important air pollutant impacting forests of the northeastern United States, including Pennsylvania. Spatial and temporal patterns of ambient, ground-level ozone were studied during 2002–2004 within north-central Pennsylvania hardwood forests. Ground-level ozone was monitored at 20 remote, forested sites using passive (non-electric) ozone samplers. Ten monitoring sites were established at (relatively) low-elevation (<350 m) locations in valleys and ten sites were located at (relatively) high-elevation locations (>550 m) on mountains. Real-time electronic ozone analyzers were co-located with the passive samplers at three sites that had access to electricity. Spatial maps were developed illustrating gradients of ozone across the region. During all 3 years, ambient ozone levels were positively correlated with elevation (2002, ρ = 0.813, P < 0.001; 2003, ρ = 0.877, P < 0.001; and 2004, p = 0.518, P < 0.019). Native forests at higher, mountainous sites may be at risk from higher ambient levels of ozone, despite their perceived “pristine” location. Future field surveys, designed to evaluate ozone injury to native vegetation, will use spatial maps developed from this study.