Although splenomegaly is major characteristic of primary myelofibrosis (PMF), splenomegaly has been devalued due to a less reliable method based on physical examination (PEx). We evaluated whether spleen volume (SV) on CT would accurately predict clinical outcomes in PMF. A total of 188 patients were enrolled. SV was quantitated by the automatic volume software. In ROC curve, the SV predicted prognosis more accurately than spleen length by PEx (p < 0.001). The ideal cut-off value was 378.1 cm(3) for SV, which was divided into high- and low-volume status. Patients with low SV status had superior leukemia-free survival and overall survival compared to high SV status (p < 0.001, p < 0.001) In the Cox analysis, old age aeyen65 years (p = 0.004, p = 0.001), low Hemoglobin < 10.0 g/dL (p = 0.023, p = 0.021), high WBC counts aeyen25 x 10(9)/L (p = 0.003, p = 0.006), peripheral blasts aeyen1 % (p = 0.029, p = 0.020), unfavorable cytogenetic abnormalities (p = 0.025, p = 0.028), and high SV status (p = 0.004, p = 0.003) were independently associated with survivals. SV measured by CT was important for predicting survival in patients with PMF.