[I-123]-iodo-L-phenylalanine was successfully evaluated for gamma camera imaging in vivo in tumor-bearing athymic mice and in humans with brain tumors. Here, we report the use of this tracer in two dogs with synovial cell sarcoma of the tarsus. [I-123]-iodo-L-phenylalanine was quantitatively prepared as a kit formulation using the Cu1+-assisted nucleophilic exchange. Rapid [I-123]-2-iodo-L-phenylalanine tumor accumulation was observed with good tumor to background contrast and rapid clearance in these two dogs. This radiopharmaceutical is a promising alternative tumor tracer to overcome the known limitations of F-18-fluorodeoxyglucose and, when labelled with radioiodine-131, has the potential to be used for therapeutic purposes.