Background: Unique to modern immune therapy for cancer is that early in the course of treatment, patients frequently exhibit transient tumor enlargement (pseudo-progression, pPROG) due to tumor infiltration by lymphocytes (TIL). Currently, pPROG cannot be reliably distinguished from true tumor progression (PROG). There is a need for biomarker techniques to discriminate pPROG (effect of therapy) and PROG (therapy failure). Nuclear medicine offers radiopharmaceuticals capable of imaging immune cells; images can be fused to evaluate functional and anatomic characteristics of tumors, and potentially discriminate pPROG from PROG. Methods: In our study of metastatic melanoma patients, SPECT/CT imaging with ^99mTc-interleukin-2 (^99mTc-IL2) was performed to visualize TIL. Images were collected before/after 12 weeks of ipilimumab (IPI) or pembrolizomab (PEMBRO) therapy. The ^99mTc-IL2 tracer was synthesized by conjugating succinimidyl-6-hydrazinopyridine-3-carboxylate (HYNIC-NHS) with commercial interleukin-2 (Aldesleukin). HYNIC-IL2 was incubated with tricine, ^99mTcO₄^- (370-740 MBq) and SnCl₂. After labelling ^99mTc-IL2 was purified by reverse-phase chromatography and diluted in 5% glucose with 0.1% human albumin before injection. Five patients were enrolled in this study. Two patients failed to complete the 12 week ^99mTc-IL2 scan due to discontinuation of IPI after: 1) grade 3 colitis and 2) patient refusal for adverse events attributed to IPI. No adverse events attributable to the tracer infusion were reported. Results: Metastatic lesions could be visualized by the tracer. Some lesions decreased in size, while others increased. A positive correlation was found between size and ^99mTc-IL2 uptake, before and after therapy, suggesting the potential discrimination of tumor PROG (no ^99mTc-IL2 uptake) from pPROG (high ^99mTc-IL2 uptake). Immunohistochemical staining for TIL of ^99mTc-IL2 positive and negative lesions are illustrated. Conclusions: The results demonstrate feasibility of ^99mTc-IL2 imaging as a clinically useful tool capable of discriminating tumor PROG from pPROG. A clinical validation study is in progress. Clinical trial information: NCT01789827