To examine factors that influence changes in Micropolyspora faeni (MF) antibody titer in farmer's lung disease (FLD), we followed for 5 yr the prevalence of serum MF antibody and the epidemiologic factors (years on farm, hours in barn, and hay-handling time) among 92 dairy farmers in Hokkaido, Japan. The prevalence of MF antibody among nonsmokers was significantly higher than that among smokers: 27.1% versus 7.7% in 1979, 31.3% versus 2.9% in 1984. There was no remarkable change in overall prevalences of MF antibody between 1979 and 1984 (18.5 and 19.6%, respectively). However, out of 17 seropositive farmers, six (35%) became seronegative, and out of 75 seronegative farmers, seven (9.3%) became seropositive after 5 yr. The nonsmoking farmers who remained seronegative throughout the follow-up period were older and had worked longer on farms than the farmers with seroconversion. These results suggest that in addition to smoking habits, age and exposure time to MF influence the immune response to MF in dairy farmers. Out of 11 farmers who remained seropositive throughout the 5-yr period, two (18.2%) developed FLD. Therefore, continuously positive MF antibody is one of the risk factors in the development of FLD.