Early detection of cancer is essential for effective intervention. Urine has been used to reflect early changes in various tumor-bearing models. However, urine has not been used to predict whether tumors will form in animal models. In this study, a cancer model was established by tail vein injection of 2 million NuTu-19 tumor cells. Urine samples were randomly selected from tumor-forming and non-tumor-forming rats on day 0/12/27/39/52 and were analyzed by label-free and parallel reaction monitoring targeted proteomic quantitative analyses. In tumor-forming rats, differential proteins were associated with tumor cell migration, TGF-β signaling and the STAT3 pathway. A total of 9 urinary proteins showed significant changes in the early phase of lung tumor formation in all eight tumor-bearing rats. Differential proteins in non-tumor-forming rats were associated with glutathione biosynthesis, IL-12 signaling and vitamin metabolism. A total of 12 urinary proteins changed significantly in the early phase in all seven non-tumor-forming rats. Our small-scale pilot study indicated that (1) the urinary proteome reflects early changes during lung tumor formation and that (2) the urinary proteome can distinguish early tumor-forming rats from non-tumor-forming rats. The lungs are common sites for cancer metastasis because of their specific immunologically permissive environment 1. Therefore, various cancers, such as breast cancer, skin melanoma, colorectal cancer, sarcoma and pancreatic cancer, metastasize more easily to the lungs than to other tissues 2. Lung metastasis is a lethal determinant in many cancers 3. The prognosis of patients with malignant lung tumors is very poor, with a low 5-year survival rate, and immunotherapy and chemotherapy have had limited success in reversing lung cancer progression 4,5. Therefore, there is an urgent need to identify biomarkers for the early detection of lung tumors and even for the prediction of whether tumors will form in the lungs. The discovery of such biomarkers will enable the development of effective therapies for metastatic lung cancer patients. Urine, as the filtrate of the blood, does not need to remain stable in composition; therefore, it is an ideal biomarker source, accumulating molecules that reflect all changes throughout the body, possibly even early and small pathological changes 6,7. Urinary proteomics analysis has already been applied for the clinical detection of lung cancer in patients 8,9. However, the urinary proteome is easily affected by various factors, such as age, sex, diet and medication, especially in the context of clinical urine samples 7. Experiments using animal models will help to minimize these influential factors and will establish the direct relationships between diseases and corresponding changes in urinary proteins 10. In addition, the exact starting point and the entire tumor progression period can be controlled in animal model studies, making it possible to collect urine samples in early stages. In our previous studies, we used urinary proteomes...