OBJECTIVE: The objective of this study was to analyze the uptake of rubidium in malignant tumors. PARTICIPANTS AND METHODS: Sixteen malignant lesions were included. Two radiologists compared each lesion to four references (subcutaneous fat, lung, mediastinal blood pool, and liver) at rest and stress and scored as 1-4. Maximum standardized uptake value (SUV) in each lesion and four references, as well as ratios of lesion SUV to SUV of each of the references, were calculated at rest and stress. We assessed an agreement for scores of reader 1 versus reader 2 (inter-reader) at rest and stress, scores at rest versus stress (intrapatient) for reader 1 and reader 2, and lesion SUV and respective ratios at rest and stress using paired t-test and Bland-Altman analyses. RESULTS: Fifteen (94%) out of 16 lesions had a score of 3 or 4 at rest or stress or both by at least one reviewer. We did not find evidence of inter-reader bias at rest or stress or intrapatient (rest vs. stress) bias for either reader. SUV ranged from 1.0 to 8.1 at rest and from 0.7 to 6.7 at stress. There was an excellent agreement between ratios of lesion SUV to lung SUV at rest versus stress. On the extreme, there was a poor agreement between ratios of lesion SUV to liver SUV at rest versus stress. Otherwise, the agreement was good for the majority of the results, and moderate for a few others. CONCLUSION: Malignant tumors can be readily depicted and quantified on rubidium PET/CT. Further research is needed.