The long-term lung retention fraction of insoluble inhaled aerosols--'alveolar deposition' (AD)--may be assessed in healthy subjects from lung retention at 24 h. For persons inhaling long-lived hazardous particles this represents their long-term dose commitment; in clinical tests of mucociliary function with inert particles it represents the fraction of particles not available to act as tracers of mucus transport. AD (24 h retention) was measured with 99Tcm-labelled, 5 microns diameter polystyrene particles which were inhaled at various flow rates; a post-inspiratory breath-hold of 3 s maximised AD efficiency. In 32 tests on young non-smokers (age less than or equal to 30 y), AD was significantly related to flow rate (r = 0.61, p less than 0.001). Intersubject variability about the regression line was not related to variability of airway calibre as detected by conventional spirometry. Significant dependence of AD on flow rate but not lung function was found in young asymptomatic cigarette smokers and in older non-smokers. In older smokers (age greater than 30 y) dependence on flow rate was significant but dependence on lung function equivocal. Smoking was associated with decreased AD. A considerable intersubject variability of AD could not be attributed to variability of inhalation conditions or lung function. AD should therefore be directly measured whenever possible in occupational hygiene assessments and clinical aerosol applications.