Persistent obstruction was assessed in 630 asthmatic subjects by measurement after bronchodilator of the maximum potential peak flow rate and forced vital capacity. Persistent obstruction was directly related to duration of asthma. The fit was slightly better after logarithmic transformation, suggesting that the early stages of asthma may be as important as the later ones in the development of persistent obstruction. On multivariate analysis age was an additional factor in males, but made no significant difference to females. Atopy did not prove relevant. Measurements of peak flow suggested a more rapid deterioration in males of social classes III, IV and V than classes I and II. Although current smokers had lower peak flow rate and vital capacity than non-smokers, there was no evidence of a steeper decline in smokers than non-smokers suggesting that the effect of cigarette smoking was at most additive to that of duration of asthma.