Farmer's lung incidence in Ireland was constant until 1996, even though hay making methods were revolutionised in late 1980's. We undertook this study to find out the incidence of farmer's lung in Ireland from 1982-2002 and its correlation with rainfall and the effect of changing farm practices. The primary cases of farmer's lung were identified from Hospital in Patients Enquiry (HIPE) unit of the national Economic & Social Research Institute (ESRI) Dublin. Rainfall data were obtained from Met Eireann whereas population, hay production and silage production were obtained from the Central Statistics Office, Dublin. As the farming population is in decline, we used the annual working unit (AWU), which reflects the true population at risk. An AWU is the equivalent of 1800 hours per farm worker per year. The incidence rates were constant from 1982-1996, but from 1997-2002 a marked decline was observed. There was strong positive correlation with hay production (r = 0.81) and strong negative correlation with silage production (r = -0.82). This study indicates that the incidence of farmer's lung is now in decline.