In a series of 27 patients a significant mean decrease of forced expiratory volume in one second (FEV1) was recorded at 1/2, 2, 4 and 24 hours after lymphography with an oily contrast medium, with a maximum mean decrease at 4 hours after injection. The decrease of FEV1 was greater in patients given large amounts of contrast medium at high injection speed. Furthermore, an excessive decrease was observed in patients with metastatic lymph nodes, while a less marked decrease of FEV1 was observed in patients treated with aspirin. No serious clinical side effects were observed. It is recommended that the injection speed and the total amount of contrast medium be limited as far as possible, especially in patients with severe airflow obstruction or with metastases in the lymphatic system. Oral administration of aspirin may limit the decrease in FEV1 following lymphography.