Concentrations of SO2 and total suspended particulates were substantially higher during the air pollution episode compared with those on all other days of the MONICA study (table 1). The episode was characterized by low temperatures, stable relative humidity, and easterly winds (12). No differences in CO concentrations were observed in comparisons of episode days with non-episode days. Total suspended particulates and SO2 were moderately correlated (r = 0.42) during the winter of 1984-1985. CO was weakly correlated with SO2 (r = 0.28) but not with total suspended particulates (r = 0.05). Three years later, the mean concentrations of SO2 had more than halved, while the concentrations of total suspended particulates and CO remained unchanged. The winter of 1987-1988 was warmer than the winter of 1984-1985. The correlation between SO2 and total suspended particulates was similar (r = 0.45), and both were correlated with CO (r = 0.45 and r = 0.52, respectively).