Cytokines are soluble factors that act as intercellular messenger between cells of the immune system. They act by binding to specific receptors that are normally poorly expressed on effector cells but in disease are upregulated following cell activation. Several cytokine/cytokine receptor systems operate in infection, autoimmunity, cancer and other pathological conditions. This enhanced receptor expression can form a target for the diagnosis of the disease process. Many cytokines have characteristics which favour their use as radiopharmaceuticals; these include a relatively low molecular weight, short half life and rapid plasma clearance, binding of high affinity to specific receptors, ready availability due to recombinant technology and lack of immunogenicity. Radiolabelled cytokines are therefore a promising class of peptide radiopharmaceuticals that may have diagnostic potential in a number of pathological conditions.