Monitoring immune function with molecular imaging could have a considerable impact on the diagnosis and treatment evaluation of immunological disorders and therapeutic immune responses. Positron emission tomography (PET) is a molecular imaging modality with applications in cancer and other diseases. PET studies of immune function have been limited by a lack of specialized probes. We identified [¹⁸F]FAC (1-(2′-deoxy-2′-[¹⁸F]fluoroarabinofuranosyl) cytosine) by differential screening as a new PET probe for the deoxyribonucleotide salvage pathway. [¹⁸F]FAC enabled visualization of lymphoid organs and was sensitive to localized immune activation in a mouse model of antitumor immunity. [¹⁸F]FAC microPET also detected early changes in lymphoid mass in systemic autoimmunity and allowed evaluation of immunosuppressive therapy. These data support the use of [¹⁸F]FAC PET for immune monitoring and suggest a wide range of clinical applications in immune disorders and in certain types of cancer.