The present invention is directed towards isomerically pure 18F-labelled (6S)- or (6R)-5-methyltetrahydrofolate radiopharmaceuticals, wherein the phenyl group within the folate structure, has been replaced by an 18F-labelled N-heterocycle, for use in diagnostic imaging of a cell or population of cells expressing a folate-receptor in vitro or in vivo or for use in monitoring of cancer or inflammatory and autoimmune diseases and therapy thereof.