The amino acid incorporation rate, generally described as protein synthesis rate or PSR, can be assessed in vivo using carboxylic-labelled amino acids such as L-[1-11C]tyrosine and PET. In animals, labelled tissue metabolites are below 4% of total tissue radioactivity and are therefore neglected in the model. Labelled plasma metabolites on the other hand rise continuously to 50% of total plasma radioactivity at 40 minutes. After correction of the total plasma radioactivity for the metabolite fraction, a Patlak analysis may be performed to calculate the PSR. A number of applications in the field of oncology were presented. The use of L-[1-11C]tyrosine in the study of metabolic disease was also discussed. It is concluded that the application of [11C]tyrosine-PET in a clinical setting is of interest for an increasing number of diseases.