We report here Phase I clinical-trial studies of retroviral-mediated interleukin-2 (IL-2) gene transfer to tumor-infiltrating lymphocytes that are re-infused to advanced lung cancer patients with pleural effusions. Ten lung cancer patients with malignant pleural effusions for whom all conventional therapy had failed were included in this Phase I protocol. Tumor infiltrating lymphocytes (TIL) from the patients were exposed to the retroviral plasmid pL(IL-2)SN containing the human IL-2 gene. Approximately 1-6 x 10(10) TIL cells transfected with IL-2 were re-infused into the chest cavity of each patient. The toxicity of this treatment with TIL/IL-2 gene therapy in these patients was minimal with transient slight fever of approximately 37.5. Pleural effusions did not re-accumulate for at least 4 weeks in six of ten patients. One patient was observed to have not only the resolution of the pleural effusions, but in addition the size of the original tumor decreased as seen by CT. The clinical results indicate this method of cancer gene therapy is safe and possibly efficacious against pleural effusions due to advanced lung cancer.