A recent phase I study of aerosolized granulocyte macrophage colony-stimulating factor (GM-CSF) in patients with malignant metastases to the lungs demonstrated excellent tolerance and possible efficacy. This therapy was offered to other patients who refused "standard" treatment or when no effective therapy was available. Forty-five patients were treated; 40 had pulmonary metastases. Aerosolized GM-CSF (250 microg/dose) was administered twice a day using a 1 week on, 1 week off schedule. The mean interval between diagnosis and therapy was 32 months. Twenty-four patients had disease stabilization or partial regression. The mean duration of benefit was 10 months. This benefit was noted in 8 of 13 with a sarcoma, 6 of 14 with melanoma, and 5 of 12 with renal cell carcinoma. Eighteen patients reported mostly self-limiting toxicities. The frequency of certain melanoma-specific T lymphocytes in 1 patient with stable disease was found to have increased 10-fold after therapy. Aerosolized GM-CSF appears to have limited but promising efficacy in treatment of pulmonary metastatic disease. In one patient, we have evidence of upregulation of melanoma-specific cytotoxic T-cells. Further study is warranted to understand the impact of this therapy on the natural history of metastatic cancer.