The lungs of twenty three subjects, 12 non-smokers and 11 smokers, age range 19-81 years, were studied. All were free from cardiopulmonary disease. The cross sectionally cut muscular pulmonary arteries of each subject were measured in sections obtained from 12 representative tissue blocks using a digitiser. Intimal area was measured and artery size defined as total length of internal elastic lamina (IEL). Intimal abnormality was expressed in the form of an intima index calculated by dividing intimal area by the area enclosed by the IEL in its theoretically unwrinkled state. Mean intima indices were calculated for arteries in four size groups: less than or equal to 600 micron, 601-1200 micron, 1201-1800 micron and greater than 1800 micron length of IEL. For all subjects intimal abnormality was most severe in the smallest muscular pulmonary arteries; values for mean intima indices decreased with increasing size of artery. There were no lobar differences. The amount of intimal abnormality increases with age in all sizes of artery: in the 600 micron size group this could average a 32% reduction in lumen calibre in those aged 60 or more. We were unable to detect any correlation between intimal abnormality and the subject's smoking history.