Ventilatory function, cellular composition in bronchoalveolar lavage (BAL) fluid and release of chemical mediators from leucocytes were examined in 25 older asthmatics (mean age 66.7 years) and 25 younger asthmatics (mean age 45.1 years) in relation to clinical asthma types. 1. Ventilatory function tests showed that the values of ventilatory parameters such as % MMF, % V(50) and % V(25) were lower in older subjects than in younger subjects, and a significant difference was found in the % V(50) value between the two age groups. Regarding clinical asthma types, the values of % MMF, % V(50), and V(25) were significantly lower in both younger and older subjects with type II. 2. In analysis of cellular composition in BAL fluid, the proportion of BAL lymphocytes was significantly lower in type II older subjects than in younger subjects with the same type. A significantly increased proportion of BAL neutrophils was observed in both younger and older subjects with type II. 3. The release of leukotriene C(4) (LTC(4)) from leucocytes was significantly lower in type II older subjects than in younger subjects with the same type, and LTC(4) release in patients with type II was also significantly lower than that in those with other asthma types in both younger and older subjects. The release of histamine and leukotriene B(4) (LTB(4)) from leucocytes was not significantly different between the two age groups. These results demonstrate that ventilatory function, cellular composition in BAL fluid, and the release of chemical mediators from leucocytes are affected by aging and clinical asthma types.