We measured the pulmonary clearance of 99mTc-labelled diethylene triamine penta-acetate (99mTc-DTPA) for 3 h in 17 non-smokers and in 16 healthy smokers. We found the clearance of 99mTc-DTPA to be well described by a mono-exponential equation in 14 non-smokers, the half-life being 66 +/- 17 min (mean +/- SD). In all smokers, a bi-exponential equation yielded a significantly better curve fit. The half-life of the slow and fast clearance components was 83 +/- 19 and 13 +/- 4 min, respectively. The relative amount of radioactivity cleared by the fast component was 57 +/- 15% and correlated significantly with cumulated tobacco consumption (r = 0.58, P < 0.02) and forced expiratory volume in 1 s in percentage of predicted value (r = -0.60, P < 0.02). We conclude that smoking induces a rapidly clearing pool of 99mTc-DTPA in the lung, the size of which may be related to smoking habits.