The Weekly Returns System of the Royal College of General Practitioners was used to assess the effect on respiratory illness of the acid transport event that occurred during January 1985. The pollution event, as assessed by SO2 and smoke levels measured at pollution monitoring stations within and without the affected area showed only modest rises in SO2 levels, which were less than levels that occurred 4 years earlier. January is the peak time of year for reporting of acute respiratory episodes, and the minor increase in pollution was not reflected in any rise in respiratory morbidity, both for all ages and for different age bands. There was a rise in rates for children up to the age of 14, but this was seen each year and in both polluted and nonpolluted areas. This was probably due to children returning to school after the winter vacation and the subsequent spread of viral infections. The limitations of the two data sets in this analysis are discussed, including the relative insensitivity of weekly data in picking out a short-lived event, the distribution of the practices and pollution monitoring stations, and the effect of the extreme cold weather and the coal miners' strike on domestic coal burning during this event.