Background: Patients with cancer of unknown primary account for 10% of patients with metastatic spinal cord compression (MSCC). This retrospective study was performed to identify prognostic factors for functional outcome, local control of MSCC, and survival in 175 of such patients treated with radiotherapy alone.Methods: Investigated were nine potential prognostic factors including age, gender, Eastern Cooperative Oncology Group performance score (ECOG-PS), number of involved vertebrae, pre-radiotherapy ambulatory status, other bone metastases, visceral metastases, time developing motor deficits before radiotherapy, and the radiation schedule.Results: On multivariate analysis, better functional outcome was associated with absence of visceral metastases (estimate: 0.72; 95%-confidence interval [CI]: 0.07-1.36; p = 0.030) and a slower (>7 days) development of motor deficits (estimate: 1.93; 95%-CI: 1.18-2.68; p < 0.001). Improved local control of MSCC was associated with absence of visceral metastases (risk ratio [RR]: 10.26; 95%-CI: 2.11-74.73; p = 0.004). Improved survival was associated with favorable ECOG-PS (RR: 2.12; 95%-CI: 1.40-3.29; p < 0.001), being ambulatory prior to radiotherapy (RR: 1.98; 95%-CI: 1.40-2.81; p < 0.001), absence of visceral metastases (RR: 2.74; 95%-CI: 1.93-3.91; p < 0.001), and slower development of motor deficits (RR: 1.27; 95%-CI: 1.07-1.51; p = 0.007). Absence of other bone metastases showed a trend (RR: 1.38; 95%-CI: 0.98-1.95; p = 0.07).Conclusions: This study identified additional independent prognostic factors for functional outcome, local control of MSCC, and survival after radiotherapy of MSCC from cancer of unknown primary. These prognostic factors can help select the best treatment regimen for each individual patient.