Purpose: (4S)-4-(3-[18F]fluoropropyl)-l-glutamate (BAY 94-9392, alias [18F]FSPG) is a new tracer to image xC− transporter activity with positron emission tomography (PET). We aimed to explore the tumor detection rate of [18F]FSPG in patients relative to 2-[18F]fluoro-2-deoxyglucose ([18F]FDG). The correlation of [18F]FSPG uptake with immunohistochemical expression of xC− transporter and CD44, which stabilizes the xCT subunit of system xC−, was also analyzed. Experimental Design: Patients with non–small cell lung cancer (NSCLC, n = 10) or breast cancer (n = 5) who had a positive [18F]FDG uptake were included in this exploratory study. PET images were acquired following injection of approximately 300 MBq [18F]FSPG. Immunohistochemistry was done using xCT- and CD44-specific antibody. Results: [18F]FSPG PET showed high uptake in the kidney and pancreas with rapid blood clearance. [18F]FSPG identified all 10 NSCLC and three of the five breast cancer lesions that were confirmed by pathology. [18F]FSPG detected 59 of 67 (88%) [18F]FDG lesions in NSCLC, and 30 of 73 (41%) in breast cancer. Seven lesions were additionally detected only on [18F]FSPG in NSCLC. The tumor-to-blood pool standardized uptake value (SUV) ratio was not significantly different from that of [18F]FDG in NSCLC; however, in breast cancer, it was significantly lower (P < 0.05). The maximum SUV of [18F]FSPG correlated significantly with the intensity of immunohistochemical staining of xC− transporter and CD44 (P < 0.01). Conclusions: [18F]FSPG seems to be a promising tracer with a relatively high cancer detection rate in patients with NSCLC. [18F]FSPG PET may assess xC− transporter activity in patients with cancer. Clin Cancer Res; 18(19); 5427–37. ©2012 AACR.