Background and purpose - Soft-tissue sarcoma (STS) is rare, with challenging individualized treatment, so diagnostics and treatment should be centralized. Historical controls are sometimes used for investigation of whether new diagnostic or therapeutic tools affect patient outcome. However, as yet unknown factors may affect the outcome. We investigated prognostic factors and prognosis in 2 nationwide cohorts of patients diagnosed with a local STS during the periods 1998-2001 and 2005-2010, with special interest in finding factors lying behind possible improvement of prognosis. Patients and methods - 2 cohorts of patients with STS of the extremities or trunk diagnosed during the periods 1998-2001 and 2005-2010 were retrieved from the nationwide Finnish Cancer Registry. Detailed information was gathered from patient files. Results - Compared to first cohort, a larger proportion of patients with inadequate surgery in the second cohort received radiation therapy, and both the local control rate and the sarcoma-specific survival rate improved in the second cohort. For sarcoma-specific survival, cohort (HR =0.6, 95% CI: 0.5-0.9), age, depth, grade, and margin were significant factors in multivariate analysis. For local control, cohort (HR =0.6, 95% CI: 0.5-0.9), age, and margin were significant in multivariate analysis. Interpretation - Known prognostic factors including type of treatment did not entirely explain the secular trend of continuous improvement in prognosis in STS. This illustrates the danger of using historical controls for investigation of whether new diagnostic or therapeutic tools have an effect on patient outcome.