Approximately 3% of cats are infected with feline leukemia virus (FeLV) based on antigen tests. However, conflicting test results often occur with the same blood sample depending on whether whole blood, plasma, or serum is used and whether the test is an ELISA, IFA, PCR, or other technology, creating confusion and the opportunity for erroneous diagnosis. In a Phase 1 study, 130 FeLV+ shelter cats were tested monthly for 6 months by serology, IFA, and real-time PCR. A total of 81% of FeLV-infected cats survived the 6-month study period. The intensive serial testing in this well-defined cohort of naturally infected cats revealed that current definitions of progressive and regressive infections do not adequately describe the full spectrum of diagnostic outcomes observed. It is now abundantly clear that it is not possible to define with confidence a cat’s true FeLV infection status with a single type of test or a single point in time. Additional research, including long-term monitoring of health status and serial testing are necessary to develop a better understanding of the significance of different test result patterns and the survival of naturally infected cats. The purpose of this Phase 2 proposal is to continue the evaluation of this unique cohort of FeLV+ cats every 6 months for two additional rounds, as one component of a lifetime survival study. Each evaluation will include physical examination and a FeLV test panel using the tests found to be of most value in Phase 1.