The O3 measurements from the RAMS monitoring network in St. Louis in the upwind and downwind wind flow direction have been used to evaluate the impact of the urban plume of St. Louis. Incremental O3 formation within the urban plume compared to regional O3 background increases both in frequency and intensity from spring into midsummer and declines again in late summer and early fall. Episodic periods with substantial increments of O3 formation to the plume over background occur especially during the summer months. Aircraft measurement of O3 are used in combination with surface measurements to estimate the transport distance and width of the plume of St. Louis. The significance is evaluated of incremental O3 formation in the St. Louis plume on population exposures as well as on fumigations of field crops or forest in rural areas downwind. The results obtained from the St. Louis plume are discussed as they may apply to plumes from other cities, especially in interior regions of the United States.