The presence or absence of venous thrombosis was determined in 29 patients with an acute exacerbation of chronic obstructive lung disease by the technique of autologous platelet labelling with indium-111. Deep venous thrombosis was diagnosed in 13 patients and in nine of these patients thrombosis was located proximal to the knee--that is, in areas associated with an appreciable risk of pulmonary embolism. One patient died; pulmonary emboli were found at postmortem examination. The incidence of venous thrombosis seen in this study is high enough to suggest that these patients may benefit from prophylactic antithrombotic treatment.