We administered human interleukin (IL)-6 to rats to examine whether it is implicated in the development of pulmonary hypertension (PH). The rats injected with IL-6 developed PH as determined by the weight ratio of the right ventricle to the left ventricle+septum. This ratio decreased with the appearance of IgG anti-human IL-6 antibody. Histologically we observed in the lungs, luminal occlusion of small muscular arteries, capillaries filled with fibrin thrombi, and localized hemorrhage. IL-6 increased the number of platelets, and the number of platelets correlated with the extent of PH. Thus, it seems likely that (IL-6) is involved in the formation of PH in this model.