Eight cases of spontaneous isolated mesenteric fibromatosis (SIMF) were retrospectively reviewed. Clinical presentations included palpable abdominal mass (n = 6), abdominal pain (n = 4), gastrointestinal bleeding (n = 2) and acute abdomen (n = 1). On sonography and computed tomography (CT), eight SIMFs (size range 3 to 24 cm, mean 14.8 cm) were categorized into four morphologic patterns: well-defined inhomogeneous, well-defined homogeneous, well-defined cystic or infiltrative mesenteric mass patterns. Well-defined inhomogeneous SIMF (n = 3) was correlated with the histopathologic finding of bundles of fibroblasts with unevenly intermingled hyaline and/or myxoid degeneration areas, whereas well-defined homogeneous SIMF (n = 2) showed scarce degenerative changes. Well-defined cystic SIMF (n = 2) were ascribed to the presence of large areas of myxoid and cystic degenerations. One SIMF presented as an infiltrative mesenteric mass and the patient died 10 months after diagnosis. Both cases of cystic SIMFs showed tumor recurrences and one patient died after 84 months. The other five patients were cured by radical tumor resection. In summary, sonography, similar to CT, is also useful for evaluating SIMF with protean morphologic features, ascribing to the underlying histopathologic changes with varied degrees of hyaline, myxoid or cystic degenerations.