Small airway function was studied in 21 acromegalic patients (13 females, 8 males) and quantitated as the maximal expiratory flow at 25 and 50% of vital capacity (VEmax25, VEmax50) breathing room air, as the difference of flow at 50% of VC breathing air and 80% helium 20% O2 (delta Vmax50) and as the volume of equal flows (VisoV). In addition, lung volumes were investigated by spirometry and body plethysmography. Static lung volumes were statistically significantly increased in both male and female acromegalic patients (p less than 0.05), but RV/TLC and FEV1/FVC did not change (p greater than 0.1). VEmax25, delta Vmax50 and VisoV showed an abnormal function of small airways in nonsmoking acromegalic patients. However, smoking acromegalics did not differ significantly from 'normal' smoking subjects, suggesting that the dysfunction of small airways is not a contributory factor to mortality from the disease. Maximal expiratory flows, when related to lung volume, were reduced in acromegalic women. Abnormal small airway function in acromegaly could either be due to obstruction or, more likely, to an exaggeration of dysanaptic lung growth.