Background Stabilization or spontaneous regressions are demonstrated in more than half of patients affected by primary desmoid-type fibromatosis (DF) in retrospective studies. The objective of this phase II study was to prospectively assess the behavior of primary sporadic DT managed by active surveillance (AS). Methods This prospective, multicenter, observational study (NCT01801176) included patients ≥18 years of age with primary sporadic DF located in an extremity or the abdominal/thoracic wall. At inclusion, all patients were initially placed on AS. Follow-up was based on clinical and radiological evaluation by magnetic resonance imaging (MRI) performed at 1, 3, 6, 9, and 12 months, and then every 6 months for 3 years. The primary endpoint was progression-free survival (PFS) at 3 years according to Response Evaluation Criteria in Solid Tumors (RECIST) version 1.1, as evaluated by a Central Review Board. Results Between 2012 and 2015, 100 patients were enrolled. The female/male ratio was 8 and the median age was 34 years (interquartile range IQR 30.8–43.9). Median follow-up was 46.6 months (IQR 36.8–61.1) and the 3-year PFS was 53.4% (95% confidence interval 43.5–63.1%). At progression (48 patients), 23 patients received active treatment. Fifty-eight patients (58%) presented with spontaneous tumor regression (decrease > 0% compared with the initial size) during the first 3 months ( n  = 35, 35%) or after an initial progression ( n  = 23, 23%), of whom 26 (26%) had partial responses (PRs). The median time to PR was 31.7 months (25.3–not available). Conclusions These data support the use of AS as the primary approach to select patients with peripheral DF who require aggressive treatment.