Ten patients with severe pulmonary hypertension due to Toxic Oil Syndrome (TOS) (3 men, 7 women; mean age 27.9 +/- 11.23 yrs.) are presented. The pulmonary vessels were examined with a micromorphometric technique. All patients had intimal fibrosis of the arteries and veins. Seven also had a thrombus in different stages. All arteries were shown to have medial hypertrophy. Plexiform lexions were found in eight cases. It is concluded that TOS can produce severe pulmonary hypertension histologically undistinguishable from the primary form. TOS can be added to the list of diseases causing plexogenic arteriopathy.