In an autopsy series 12 out of 152 cases of pneumonia occurring in patients under 20 years of age were found to be Hecht (giant cell) pneumonia (8 per cent). Histological examination of 400 pneumonias from patients over 20 years of age showed no such case. Six of the cases were considered to be due to measles on the basis of eosinophilic inclusion bodies in the cytoplasm and nuclei of the giant cells, Warthin‐Finkeldey cells in the lymphoid system and on clinical data. Only one had a typical exanthema. All cases with measles were between 2 and 11 years of age. In the remaining 6 cases there was no morphological or clinical evidence of measles. All, except one, were below 1 year of age. The role of depressed immunological function in the pathogenesis of Hecht pneumonia is discussed. In the group caused by measles the depressed immunological function may partly be iatrogenic due to immunospressive treatment of leukaemic children. The other group of cases consisted of children with inborn defects some of which showed morphological signs of immunodeficiency.