AbstractThe effects of ageing on the numbers of alveolar pores of Kohn and on the cytoplasmic components of alveolar type II cells were studied in monkey lungs by scanning and transmission electron microscopy. Lung tissue from 26 female and three male pigtail macaques whose ages ranged from 1 month to 31 years (life span is 35 years) was analysed. From the age of 1 month to 10 years there was a significant increase in the number of alveolar pores (r= 0·85,p<0·001); however, between the ages of 14 years to 31 years there was no significant change. In seven animals ranging in age from 1 month to 4 years (mean 2·4 years) the number of pores was 5·8 ± 3·9 (mean · S.D.), whereas in 10 animals aged 16 to 31 years (mean 20·3 years) the number of pores was 32·7 ± 17·5 (mean ± S.D.) per alveolar profile, a significant difference (p<0·002). In older animals (15–20 years) there was a significant decrease, both in the number of lamellar bodies per alveolar type II cell (p<0·01) and in the volume density of lamellar bodies to cytoplasmic volume (p<0·05) compared with young animals (1 month to 4·8 years). In older animals, there was also a significant increase in the volume density of a vacuole‐like dilatation of the endoplasmic reticulum in alveolar type II cells (p<0·05) compared with young animals. These findings suggest impaired pulmonary surfactant production with aging. Both the increased number of alveolar pores and the postulated decrease of surfactant production could play a role in the pathogenesis of