In a group of moderately severe asthmatic subjects, regional deposition of 3.6-microns (aerodynamic diameter) monodispersed Teflon particles labeled with 111In was studied twice. The particles were inhaled with maximally deep inhalation at 0.5 l/s. Lung retention was measured at 0, 6, 24, and 48 h by use of a profile scanner equipped with two 13 x 5-cm NaI crystals. The retentions at 24 (Ret24) and 48 h were highly correlated (r = 0.96 with a slope of the regression line close to 1). There was a poor correlation between retention at 6 h and Ret24 (r = 0.54). The Ret24 values at the two exposures were well correlated (r = 0.86). There were significant correlations between airway resistance as well as single-breath nitrogen test phase III and Ret24 (r = 0.70 and 0.67, respectively). The correlation between single-breath nitrogen test phase III and Ret24 persisted also when only subjects within a narrow interval of airway resistance were included. The study indicates that regional deposition can be studied by measurements of Ret24 in subjects with moderately severe asthma and that it is dependent on changes in both large and small airways.