BACKGROUND: This study sought to determine whether 18Ffluorothymidine (FLT) positron emission tomography (PET)/computed tomography (CT) imaging allows assessment of tumor viability and proliferation in patients with soft tissue sarcomas who are treated with neoadjuvant therapy. METHODS: Twenty patients with biopsy‐proven, resectable, high‐grade soft tissue sarcoma underwent 18FFLT PET/CT imaging before and after neoadjuvant therapy. Histologic subtypes included sarcomas not otherwise specified (n = 5), malignant peripheral nerve sheath tumors (n = 3), gastrointestinal stromal tumors (n = 3), leiomyosarcomas (n = 3), angiosarcomas (n = 2), and others (n = 4). Changes in 18FFLT peak standardized uptake value (SUVpeak) were correlated with percent necrosis in excised tissue, whereas posttreatment 18FFLT tumor uptake was correlated with thymidine kinase 1 (TK1) expression and Ki‐67 staining indices in excised tumor tissue. RESULTS: Tumor FLT SUVpeak averaged 7.1 ± 3.7 g/mL (range, 1.9‐16.1 g/mL) at baseline and decreased significantly to 2.7 ± 1.6 g/mL (range, 0.8‐6.0 g/mL) at follow‐up (P < .001); however, marked reductions in SUV were not specific for histopathological response. The posttreatment SUVpeak did not correlate with TK1 (P = .27) or Ki‐67 expression (P = .21). CONCLUSIONS: Marked reductions in 18FFLT tumor uptake in response to neoadjuvant treatment were observed in most patients with sarcoma. However, these reductions were not specific for histopathologic response to neoadjuvant therapy. Furthermore, posttreatment 18FFLT tumor uptake was unrelated to tumor proliferation by Ki‐67 and TK1 staining. These results question the value of 18FFLT PET imaging for treatment response assessments in patients with soft tissue sarcoma. Cancer 2012;118: 3135–44. © 2011 American Cancer Society. In patients with high‐grade soft tissue sarcoma, 18Ffluorothymidine positron emission tomography/computed tomography imaging does not reliably predict histopathological response to neoadjuvant therapy, and posttreatment 18Ffluorothymidine uptake is unrelated to thymidine kinase 1 and Ki‐67 expression.