The aim of the study was to establish patterns of respiratory function in Northern Ireland and to examine the relationship between physical activity, physical fitness and respiratory function. We identified 1600 adults over 16 yr using 2 stage probability sampling. Physical activity was measured using a questionnaire, physical fitness from oxygen uptake while walking on a treadmill, and respiratory function using spirometry. The main outcome measures were a physical activity profile based on computer assisted interview, physical fitness by predicted VO2max, Forced Vital Capacity (FVC) and Forced Expiratory Volume (FEV1). We found that the main findings were of relationships between activity and FVC and FEV 1 which remained after adjustment for possible confounders in men, and between fitness and FVC and FEV 1 in both men and women although these were not sustained after adjustment for possible confounders.