This study examines the relationship between mesoscale meteorological conditions and high-ozone days in the northeastern United States. It is proposed that the leeside trough and the sea-breeze front are two mesoscale features that can be important on these high-ozone days. The 19 June and 4 July 1995 case studies are presented to illustrate the role of each feature, respectively. The 19 June case revealed that the leeside trough initially formed to the lee of the high terrain from southeast Pennsylvania to the Maine coast but shifted southeastward during the course of the day. As the trough advanced toward the coast, the surface winds ahead of the trough remained southwesterly, paralleling the source regions of the major metropolitan areas. It appears that the surface flow near the trough acted to elevate and concentrate ozone and its precursors in a narrow band along and just south of the trough, with much lower values north and west of the trough, where cleaner trajectories were encountered. In the 4 July case, the sea-breeze front initially formed parallel to the Atlantic coast and advanced northward through southeast New York and southern New England as the larger-scale flow became more southerly. High ozone concentrations developed in a narrow, east–west band along, and just south of, the northward-moving boundary. The combination of the boundary’s initial location near a major source region, confluent airflow along the front, and a capping inversion on the front’s cool side is presented as the likely cause for the observed pattern of high ozone. The high ozone values occurred despite relatively low peak values up and down the coast the previous day.