Early injury of the small airways has been demonstrated in asymptomatic smokers. Ventilatory tests including the maximum midexpiratory flow rate and closing volume have been useful in clinical detection of small airways disease in symptomatic subjects. In the present study, airway “obstruction” was assessed aerodynamically by gamma camera measurements of chest radioactivity following the inhalation of 131I-labeled aerosol (aerodynamic mass median diameter 3 mum). Studies were performed in normal subjects, asymptomatic smokers, and patients with chronic obstructive lung disease. An aerosol penetrance index (AeP) was devised from determinations which involved 1) an analysis of central (inner zone) and peripheral (outer zone) deposition of aerosol in the lung and 2) a ratio of initial counts to 24-h counts in the periphery (outer zone) of the lung. AeP values were 41.5 +/- 11.5 for the normal group, 20.9 +/- 7.6 for the smoker group, and 10.6 +/- 5.2 for the subjects with chronic obstructive airway disease. AeP was significantly reduced in the smokers indicating that the AeP is a sensitive index of early peripheral airways obstruction.