The determination of the prevailing site of bronchial constriction in asthma is of noteworthy interest as regards knowledge of the pathophysiological mechanisms of this disease, and of practical importance because the intrapulmonary distribution of inhaled drugs depends chiefly on airway caliber. To visualize the alteration of convective ventilation caused by the alterations of airway caliber, we developed a technique based on inhalation of a monodispersed aerosol of human albumin minimicrospheres (mean aerodynamic diameter 0.75 micron, geometric SD 1.19) labeled with 99mTc(Hamm). Intrapulmonary HAMM deposition is revealed externally by means of a gamma camera in 4 projections, soon after inhalation and 4 h later, when the mucociliary activity has removed the fraction of HAMM deposited in larger airways. In normal subjects, about 16% of the inhaled HAMM deposited in airways, mostly (90% of total deposited fraction) by sedimentation in airways peripheral to the 16th generation. We obtained 58 determinations of the HAMM deposition pattern in 38 patients (17 males and 21 females, mean age 35.2 +/- 16.8 years) with symptomatic or asymptomatic asthma, none under acute attack. At the same time we obtained the determination of bronchoconstriction degree by measuring the forced expiratory volume in 1 s (FEV1) and the airway resistance (Raw) by plethysmographic technique. In 10 asymptomatic patients with normal bronchoconstriction indexes, we revealed the pattern of HAMM deposition before and after a reduction of about 30% in FEV1 in induced by bronchial challenge with carbachol.(ABSTRACT TRUNCATED AT 250 WORDS)