The long-term lung retention of 3.6 μm dia. polystyrene particles with Sr-85 was measured in five healthy non-smokers, six healthy ex-smokers, eight smokers and six persons with chronic obstructive lung disease (COLD). Normal long-term clearance was in two phases with half-times of 30±23 days and 296±98 days, respectively, 27±13% clearing via the fast phase. Half-times and relative amounts clearing via the two temporal phases were independent of particle penetration, implying that they are associated with two different mechanisms clearing particles from the same anatomical compartment. A repeat study in one person was consistent with the first, suggesting that long-term clearance is a well-defined physiological function characteristic of an individual. Cigarette smoking suppressed the fast clearance phase and increased the half-time of the slow phase by 14.7±3.0 days (95% confidence limit) per pack-year of smoking. Healthy ex-smokers had normal long-term clearance, but the individuals with COLD had significantly increased slow phase half-times.