Description: CXCR4 is a chemokine receptor that is overexpressed in various human cancers and is involved in tumor metastasis. The aim of this proof-of-concept study was to evaluate a novel CXCR4-targeted PET probe in patients with solid cancers with reported in vitro evidence of CXCR4 overexpression and to estimate its potential diagnostic value. Methods: Twenty-one patients with histologically proven pancreatic cancer, laryngeal cancer, non–small cell lung cancer, prostate cancer, melanoma, breast cancer, hepatocellular carcinoma, glioblastoma, sarcoma, or cancer of unknown primary underwent PET imaging using the novel CXCR4 nuclear probe 68 Ga-pentixafor. The SUV max of the liver, spleen, and bone marrow was measured to determine physiologic tracer distribution. For evaluation of tracer accumulation in solid cancers, SUV max and tumor-to-background (T/B) ratios were determined in a total of 43 malignant lesions, including 8 primary tumors, 3 locally recurrent tumors, and 32 metastases. When available, the SUV max of malignant lesions was compared with the corresponding SUV max measured in routine 18 F-FDG PET. Results: Moderate tracer accumulation was detectable in the liver, bone marrow, and spleen, with a mean SUV max of 3.1, 3.7, and 5.6, respectively. By visual interpretation criteria, 9 of 11 primary and locally recurrent tumors were detectable, exhibiting a mean SUV max of 4.7 (range, 2.1–10.9) and a mean T/B ratio of 2.9. Twenty of 32 evaluated metastases were visually detectable (mean SUV max , 4.5 [range, 3.2–13.8]; mean T/B ratio, 2.8). The highest signal was detected in a patient with non–small cell lung cancer (SUV max , 10.9; T/B ratio, 8.4) and a patient with cancer of unknown primary (SUV max , 13.8; T/B ratio, 8.1). Compared with 18 F-FDG PET, which was additionally performed in 10 patients, 68 Ga-pentixafor PET had a lower SUV max in all measured malignant lesions. Conclusion: On the basis of these first observations in a small and heterogeneous patient cohort, the in vitro CXCR4 expression profile of solid cancers and metastases described in the previous literature does not seem to sufficiently depict the in vivo distribution revealed by CXCR4-targeted PET. Moreover, the detectability of solid cancers seems to be generally lower for 68 Ga-pentixafor than for 18 F-FDG PET.