A 32-year-old man was admitted to our hospital complaining of abdominal pain in the left upper quadrant. A mass was palpable on the left side of the umbilicus. Laboratory data revealed anemia, elevated erythrocyte sedimentation rate, hypergammaglobulinemia, and prolonged prothrombin time. Computed tomography demonstrated a soft tissue mass in the mesentery of the jejunum, portal venous thrombosis, and cavernomatous transformation in the porta hepatis. The patient was eventually diagnosed by laparoscopic partial resection as having inflammatory pseudotumor of the mesentery. Four months later, all of his symptoms and abnormal laboratory findings completely disappeared without any therapy. Inflammatory pseudotumor should be kept in mind as a cause of portal venous thrombosis, and/or cavernomatous transformation although it is rare.