This study evaluated hepatic lesion volumes on contrast-enhanced computed tomography (CT) images in patients with pyrrolizidine alkaloid-induced sinusoidal obstruction syndrome (PA-SOS) and the association of lesion volume with the clinical severity and prognosis of the disease. Twenty-five patients with PA-SOS were included in this study, and all patients were subjected to a complete CT imaging series. The imaging results were quantitatively analyzed by a threshold-based region growing algorithm. The liver volumes and hepatic lesion volumes of the patients were estimated. Based on clinical presentations, PA-SOS was classified into three categories: mild, moderate and severe. The associations of hepatic lesion volumes with liver function test parameters and the clinical severity and prognosis of the disease were analyzed. Based on estimations using the threshold-based region growing algorithm, hepatic lesion volumes in patients with mild PA-SOS were significantly lower than those in patients with moderate and severe PA-SOS (P < 0.05). The ratio of hepatic lesion volume to liver volume (Ratio) varied based on alanine aminotransferase (ALT), aspartate aminotransferase (AST) and serum total bilirubine levels; clinical severity; and disease prognosis, and the differences were statistically significant (P < 0.05). In conclusion, the threshold-based region growing algorithm can be employed to quantitatively analyze enhanced CT images of PA-SOS patients. And the ratio of hepatic lesion volume to liver volume in patients with PA-SOS is associated with a more serious clinical course and a poorer outcome.