From a cohort of 1,768 Boston fire fighters studied initially in 1970, pulmonary function measurements were repeated on 1, 146 active subjects in 1974. The mean time between examinations was 3.4 years. The data on 1,430 subjects after one year of follow-up had indicated excessive decrements in levels of forced vital capacity and one-second forced expiratory bolume which were related to the frequency of fire exposure. The annual decline over three years was less than that observed over one year and could not be related to the number of fires fought or to other indices of acute fire exposure. Selection factors within the fire department appear to be important in protecting fire fighter from continued loss ventilatory capacity.