One hundred and eighty-seven operable lung tumours were immunostained with the monoclonal antibody Ki-67 and divided into groups of high, moderate or low proliferation. Patients have been followed clinically for up to 7 years to ascertain whether this immunocytochemical measurement reflected tumour behaviour in terms of survival. The majority of the tumours were squamous cell carcinomas (104 cases) and adenocarcinomas (60 cases). These were divided into three groups of low, intermediate and high growth fraction, in which survival was better for tumours of lower proliferative rate up to approximately 2 years after operation. By 5 years these differences had largely disappeared and all tumours of a particular type showed a similar survival curve. Small cell carcinomas (13 cases) had high Ki-67 labelling indices, with more than 60% of patients deal in the first year, whereas carcinoid tumours (10 cases) had low labelling rates and all but one are still alive. We conclude that measurement of lung tumour growth rate with the monoclonal antibody Ki-67 shows promise as a possible indicator of short-term survival and perhaps as a means of choosing a group of patients with adenocarcinomas and squamous cell carcinomas for post-operative chemotherapy.