Background. We experienced a rare case of a pregnant woman with Klippel-Trénaunay syndrome complicated with diffuse venous malformation of the uterus. This is the first report on the usefulness of dynamic contrast-enhanced-MRI for the diagnosis of diffuse venous malformation of the uterus. Case Presentation. A 23-year-old woman presented with convulsions and talipes equinus position of both lower limbs at 11 weeks of gestation. At 27 weeks, ultrasonography demonstrated tubular echolucent spaces throughout the myometrium. Dynamic MRI at 37 weeks revealed that the myometrial lesion was enhanced slowly and showed homogeneous enhancement even on a 10 min delayed image. Taken together with unilateral foot hypertrophy, varices, and port-wine stain, the patient was diagnosed as having Klippel-Trénaunay syndrome complicated with diffuse venous malformation of the pregnant uterus. The patient underwent elective cesarean section because of severe dystonia. The lower uterine segment was thickened and heavy venous blood flow was observed at the incision. Histological diagnosis of the myometrial biopsy specimen was venous malformation. Conclusions. Both diffuse venous malformation and Klippel-Trénaunay syndrome during pregnancy can involve considerable complications, in particular, massive bleeding during labor. Women who suffer from this syndrome should be advised about the risk of complications of pregnancy.