This report presents the results of an ozone monitoring experience performed with tobacco indicator plants (Nicotiana tabacum L. cv. 'BeI-WY). These bioindicators were exposed to ambient air in 23 sites in Northern Italy from 27 April 1987 to 21 October 1987. Typical ozone-induced leaf necrosis was observed in every site. The percentage increase in injured leaf area was estimated weekly and a Leaf Injury Index (LII) was calculated. The weekly series of LIIs were statistically inspected for spatial and temporal relationships. Sample correlation coefficients were statistically significant for almost every possible pair of sites. The time series properties of ten series of LIls with no missing data were analyzed using Box-Jenkins models. An autoregressive first order model, or AR(1), was selected to remove the autocorrelation from these series. Linear correlation coefficients between 'prewhitened" (i.e. from which autocorrelation had been removed) pairs of LII series were statistically significant. Hence synchronous variations in leaf necrosis could only be related to the regional diffusion of tropospheric ozone. * Work carried out as part of the research programme: 'ENEL's contribution to the knowledge of important environmental issues', Research line: 'Effects of atmospheric pollution on the land'.