The differences in microscopic appearance which characterize the main histological types of lung cancer suggest associated differences in biological behaviour. For example, their growth rates might be expected to increase as the degree of cell differentiation decreases, in which case the least differentiated tumours would be the most malignant.One method of assessing the relative behaviour of different types of tumour is by studying the survival rates of untreated patients in whom they occur. However, among untreated patients with lung cancer the course of the disease is so short that survival differences between the histological types are small. A more useful index of relative biological behaviour is provided by comparative survival rates after treatment. Many papers have been written about this, mainly on the relation between tumour histology and survival after surgery, and these have been reviewed by Bignall (1958). From the data included in this review, and from other more recent publications, several apparent associations emerge.